Merge with pypa/distutils@a7cfb56a7

[jaraco]

• Detect when a venv is created from an in-tree build. Fixes Missing license #132
• make all fields documented as optional in the spec truly optional (dont warn when omitted), and stop coercing missing values to the string "UNKNOWN"
• Extract check for 'd is None'.
• Combine logic between prefix and base_prefix.
• Adapt style
• Simply emit the expected value instead of converting to an exit code and deciphering.
• Rely on jaraco.envs for environment creation.
• Prefer tabs
• Use pass_none decorator.
• Prefer tox 3.25 with built-in support for Windows env vars.
• Run tests using tox, even on cygwin.
• Prefer tabs
• address review comments
• fix a typo
• Update changelog

Summary of changes

Closes

Pull Request Checklist

• Changes have tests
• News fragment added in changelog.d/.
  (See documentation for details)

[jaraco]

Tests for cygwin are failing because the "tox" command is not found. That's strange and surprising because I'm using the same technique for installing and invoking tox as is used by distutils.

[jaraco]

Apparently Python 3.9 is required for tox to get installed in the path. Not sure why.

[DWesl]

Apparently Python 3.9 is required for tox to get installed in the path. Not sure why.

The /usr/bin/tox script gets installed by each of python35-tox, python36-tox, python37-tox, python38-tox, and python39-tox in turn, but can only be a part of one of those packages. The maintainer usually chooses the last one, to match the shebang line in the script. I tend to use python -m pytest to avoid needing to know which package the script got installed in, and updating everything when it gets moved (say, when Cygwin adds python 3.10 and makes that the default for command-line scripts).

If you want to find the discussion on the mailing list, I think the script that most recently got mentioned was /usr/bin/sphinx-build moving from python3-sphinx (replaced by python36-sphinx) to python38-sphinx.