gh-59022: Added tests for `pkgutil.extend_path` (#59022)

[beachmachine]

This adds tests for the documented behaviour of pkgutil.extend_path regarding different argument types as well as for *.pkg files.

I took inspiration from the PR #12871 for the tests I've added. However, some of those tests did not seem to make sense to me, so I've ended up with the two test-cases in this PR.

• Issue: gh-59022

[ghost]

All commit authors signed the Contributor License Agreement.

[encukou]

Thanks! Just one more issue :)

For the future: please avoid force-pushing to CPython PRs; it makes them harder to review. Just add additional commits. The PR will be squashed when it's merged.

[beachmachine]

Thanks! Just one more issue :)

For the future: please avoid force-pushing to CPython PRs; it makes them harder to review. Just add additional commits. The PR will be squashed when it's merged.

Thanks for the hint. I will not use force-push for future CPython PRs.

Also, the comment in the test is removed now, so this PR is ready to be reviewed again.

[encukou]

Thank you!

[miss-islington-app[bot]]

Thanks @beachmachine for the PR, and @encukou for merging it 🌮🎉.. I'm working now to backport this PR to: 3.12. 🐍🍒⛏🤖

[miss-islington-app[bot]]

Thanks @beachmachine for the PR, and @encukou for merging it 🌮🎉.. I'm working now to backport this PR to: 3.13. 🐍🍒⛏🤖

[miss-islington-app[bot]]

Thanks @beachmachine for the PR, and @encukou for merging it 🌮🎉.. I'm working now to backport this PR to: 3.12. 🐍🍒⛏🤖

[miss-islington-app[bot]]

Thanks @beachmachine for the PR, and @encukou for merging it 🌮🎉.. I'm working now to backport this PR to: 3.13. 🐍🍒⛏🤖

[bedevere-app[bot]]

GH-121950 is a backport of this pull request to the 3.12 branch.

[bedevere-app[bot]]

GH-121951 is a backport of this pull request to the 3.13 branch.