Implement CPython 3.14 subinterpreter APIs - #8605

Argument matching:
- Add `function::ArgSpec`, a `PyArg_ParseTupleAndKeywords` equivalent that
  applies the `|`, `$` and `:` markers of a format string to a `kwlist`, and
  route every module function of both modules through it. This enforces
  keyword-only parameters, rejects unexpected keywords, accepts keyword forms
  that were previously positional-only (`whence(id=)`, `get_config(id=)`,
  `set___main___attrs(id=, updates=)`, `capture_exception(exc=)`,
  `_register_end_types(send=, recv=)`), and reproduces the arity messages.
- `O!` slots (`shared`, `updates`, `call`'s `args`/`kwargs`) now reject None,
  and `_PyArg_BadArgument` renders None as "None" rather than "NoneType".
- `new_config` takes at most one positional `str`; `list_all` of
  `_interpchannels` is argument-less.
- Convert arguments in `kwlist` order, so an argument's own error is raised
  ahead of the checks that follow it.

_interpreters:
- Add the `CrossInterpreterBufferView` type and build the received memoryview
  on it, keeping the sending interpreter's exporter out of the destination.
- Build `excinfo.errdisplay` from `traceback.TracebackException`.

Feature flags:
- `os.fork` checks finalization before `allow_fork`, matching `os_fork_impl`.
- `_thread.start_new_thread` and `start_joinable_thread` reject an interpreter
  without `allow_threads`.

Lib/concurrent/interpreters/_crossinterp.py: restore the stripped docstrings
and comments.

Assisted-by: Claude:claude-opus-5

…t CPython 3.14

verify_stateless_function follows _PyFunction_VerifyStateless: it rejects
non-dict builtins, a non-empty __defaults__, __kwdefaults__ or __closure__,
and code whose LOAD_GLOBAL names are held by the function's globals or
absent from its builtins.

code_returns_only_none follows _PyCode_ReturnsOnlyNone: generator,
coroutine and async-generator code is rejected up front, the instruction
walk skips inline caches and maps specialized and instrumented opcodes
back, and the LOAD_CONST preceding each RETURN_VALUE is compared against
the index of None in co_consts.

ExcInfo::capture keeps an empty exception message instead of dropping it,
and builds `formatted` as `module.qualname: msg`, leaving out the builtins
and __main__ modules.

_interpreters.call packs the callable through _PyFunction_GetXIData before
pickle and re-raises the stateless-check failure when both fail. A func,
args or kwargs that cannot be rebuilt in the target, and a result without
cross-interpreter data, now raise NotShareableError with the snapshot as
the cause rather than being returned as excinfo.

_interpreters.capture_exception() without an argument returns None.

pickle_loads and _PyFunction_GetXIData attach the underlying failure as
__cause__.

channel_send converts the object to cross-interpreter data after the
channel's `closing` check.

ChannelID rich comparison returns the result of comparing its id with the
other number instead of coercing that result to bool.

PyMemoryView_FromObjectAndFlags raises "memoryview: a bytes-like object is
required, not 'X'" for an object that is not a buffer.

Assisted-by: Claude:claude-opus-5

Bound shareable ints by isize, raising OverflowError("try sending as
bytes") outside that range and keeping it as the NotShareableError cause.

Report an unshareable object by its repr, and attach a failing conversion
as __cause__ of the NotShareableError raised for the object it was called
for, so each level of a nested tuple appears in the chain.

Guard each tuple item conversion with with_recursion("while sharing a
tuple").

Gate the memoryview getdata function on `_interpreters` having been
imported, and route channel_send_buffer through a memoryview so it uses
that function and the resolved fallback.

Create the cross-interpreter exception types from the module_exec of
either module that raises them, instead of only `_interpreters`.

parse_cid raises OverflowError("int too big to convert"); int_arg
reproduces the `i` converter's three overflow messages and is also used
while parsing channel_send arguments.

Add BASETYPE to _queue.Empty, and gate nt.execv/nt.execve on allow_exec.

Assisted-by: Claude:claude-opus-5

_PyInterpreterState_ObjectToID accepts any object with __index__, raises
OverflowError("int too big to convert") outside the int64 range, and
reports a negative ID with the repr of the original object.

channelsmod_send reads the channel's default unboundop and fallback only
when one of the two arguments is negative, so an explicit pair is
validated before the channel is looked up.

Assisted-by: Claude:claude-opus-5

_interpqueues holds a process-wide queue table and exposes create,
destroy, list_all, put, get, bind, release, get_maxsize,
get_queue_defaults, is_full, get_count and _register_heap_types, raising
QueueError and QueueNotFoundError.  QueueEmpty and QueueFull, which
subclass queue.Empty and queue.Full, are registered per interpreter by
concurrent.interpreters._queues.

Cross-interpreter data carries a queue as a refcounted queue ID that
binds when the data is captured and releases when it is dropped, and
is_shareable reports a registered Queue as shareable.

Queue and channel items owned by an interpreter are now cleared from
Interpreter::finalize, after module finalization, rather than from
destroy_owned_interpreter, so values sent by an atexit callback also
become unbound.

is_shareable gates memoryview on the same registration flag as the
getdata lookup.

Assisted-by: Claude:claude-opus-5

Interpreter, ExecutionFailed and the queue aliases, copied verbatim from
CPython 3.14.

Assisted-by: Claude:claude-opus-5

Interpreter::finalize now runs finalize_subinterpreters between the atexit
handlers and the finalizing flag: when the main interpreter still owns
subinterpreters it emits the RuntimeWarning "remaining subinterpreters;
close them with Interpreter.close()" and then finalizes each of them.

runtime::owned_interpreter_ids lists the runtime-owned interpreters.

Assisted-by: Claude:claude-opus-5

_Py_abspath and _PyPathConfig_ComputeSysPath0 arrive as abs_path and
script_sys_path0. run_file hands the absolutized path to get_importer
and to the runner, so __file__ and co_filename no longer stay relative,
and inserts the symlink-resolved script directory as sys.path[0]
instead of the argument's parent.

test_cmd_line_script.test_script_abspath no longer expects a failure.

Assisted-by: Claude:claude-opus-5

The object, number, sequence and mapping protocols, the
unsupported-operand helpers, and the type-specific messages fed from
them now read PyType::slot_name() where they read PyType::name(),
so a type declared with a module keeps it.

Message texts corrected along the way:
- PySequence_Size, PyMapping_Size, PySequence_GetItem and
  PySequence_SetItem/DelItem split their merged wording in two, and
  PyObject_SetItem/DelItem say "object".
- PyObject_GenericSetAttr reports a read-only attribute and the
  missing __dict__.
- check_class drops the type name it appended.
- __int__ and __index__ drop the class prefix; __float__ keeps it.
- float(), complex(), round(), next(), str subscripting and list
  subscripting take their own wording.
- str.join and bytes.join number the item they turn down and name what
  they wanted, and report a non-iterable as "can only join an
  iterable".

test_class.testObjectAttributeAccessErrorMessages,
string_tests.test_subscript and the test_descrtut doctest no longer
expect a failure.

Assisted-by: Claude:claude-opus-5

crossinterp::script_code called vm.compile unconditionally, which only
exists with the rustpython-compiler feature. Builds without it, such as
example_projects/barebone, now raise a TypeError instead of failing to
compile.

Assisted-by: Claude:claude-opus-5

The test imports _testsinglephase in its body without a decorator
guarding it, and now that _interpreters exists requires_subinterpreters
no longer skips it.

Assisted-by: Claude:claude-opus-5