Handle unreadable configuration files - #4035

Fixes #4031.

The problem

The three source-loading paths in tox.config.source.discover translate ValueError into HandledError, but reading a file can also raise an OSError subclass. So a config file that exists and cannot be read lets PermissionError escape:

$ chmod 000 tox.ini
$ tox -c tox.ini list
Traceback (most recent call last):
  ...
PermissionError: [Errno 13] Permission denied: 'tox.ini'

After:

$ tox -c tox.ini list
ROOT: HandledError| ToxIni failed loading /tmp/p/tox.ini due to [Errno 13] Permission denied: 'tox.ini'

The change

OSError is caught alongside ValueError in all three paths — the directory branch of discover_source, _locate_source, and _load_exact_source — so the named file, automatic discovery, and -c <directory> all behave the same. Verified all three by hand.

A note on the test

The regression test exercises discover_source directly rather than going through the tox_project fixture.

I wrote it through the fixture first, and it passed against unfixed code. The fixture converts exceptions on its own, so from the outside both versions produce the same HandledError| ToxIni failed loading line and the test cannot tell them apart. Testing the function where the bug lives does distinguish them: unfixed it raises PermissionError, fixed it raises HandledError.

The test is skipped on Windows and when running as root, since neither can make a file unreadable this way.

tests/config/source/: 200 passed.