packaging: include tox.ini in source distribution (sdist)

[pajod]

• Fixes: #3267 (not immediately, the change merely requests inclusion in future source distributions)

Argument from authority: See what setuptools does & documents: https://github.com/pypa/setuptools/blob/1d2fd37368fc2381e21cd5466aeb222fc11db2db/MANIFEST.in

Confirmation:

$ python -m build
$ tar -tf dist/gunicorn-*.tar.gz | grep tox
gunicorn-23.0.0/tox.ini

• note: if ever using setuptools-scm: .gitattributes / MANIFEST.in precedence could undo this change
• cost is minimal, as the files are tiny and most people will download the wheel package instead.

[jayaddison]

@pajod why should tox.ini be included in a distributable? Naively I would assume that a distributable release includes only the application code -- that is, the code necessary to run a project on a production system.

[jayaddison]

Sorry! This is an sdist -- a source distributable. And those can include test code. In that case it seems more reasonable to include tox.ini.