Bypass encoding warnings in execnet

[jaraco]

Originally, I just pushed this commit to main before I realized it causes failures on Python 3.8 when something attempts to construct a requirement, but creates the wrong syntax:

  File "/home/runner/work/setuptools/setuptools/.tox/py/lib/python3.8/site-packages/packaging/requirements.py", line 37, in __init__
    raise InvalidRequirement(str(e)) from e
packaging.requirements.InvalidRequirement: Expected end or semicolon (after URL and whitespace)
    execnet@ git+https://github.com/jaraco/execnet@bugfix/195-encodingwarning; extra == "testing"
             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^

The error message is misleading. The problem is that PEP 508 specifies url_req as

url_req       = name wsp* extras? wsp* urlspec wsp+ quoted_marker?

Where the urlspec needs to be separated from the quoted marker (the semicolon) with whitespace.

[jaraco]

I can see that importlib_metadata fixed this issue in 4.8.3, so there must be some other code that's generating the invalid req.

[jaraco]

The problem exists in build when it relies on the old, unfixed importlib.metadata found in Python 3.8 (fix was added to CPython 3.9 in python/cpython#90263).

[jaraco]

Although tests are passing, I don't believe PyPI would accept this package. Probably the overrides need to be in tox instead.