Update sdist rules to include tox.toml (#3389)

[mgorny]

Update sdist rules to include tox.toml, rather than tox.ini that no longer exists. This fixes test failures due to missing tox configuration.

Fixes #3389

• ran the linter to address style issues (tox -e fix)
• wrote descriptive pull request text
• ensured there are test(s) validating the fix
• added news fragment in docs/changelog folder
• updated/extended the documentation

[webknjaz]

@gaborbernat would you mind if I send a PR making the CI test from sdist that would've caught this?

[gaborbernat]

Neah, I prefer the faster behavior 99% over this rare edge case. And I do not think it would have caught it, because there still was the file locally.

[webknjaz]

@gaborbernat with my approach it would as I fully replace a Git checkout with untaring the sdist.

[gaborbernat]

Neah. Not worth it, IMHO, prefer parity between local and CI. The 1% this happens, we can push out a fix.

[gaborbernat]

https://github.com/henryiii/check-sdist I think actually would be a better approach to edit

[webknjaz]

Mine is https://github.com/re-actors/checkout-python-sdist

[gaborbernat]

I prefer Henry's solution as I find it less heavy-handed and works locally too.