Set up caching PyPI deps in GHA

[webknjaz]

Ref #1375 (comment)

Changelog-friendly one-liner: Made GHA workflows faster by enabling pip cache

Contributor checklist

• Provided the tests for the changes.
• Gave a clear one-line description in the PR (that the maintainers can add to CHANGELOG.md on release).
• Assign the PR to an existing or new milestone for the target version (following Semantic Versioning).

[atugushev]

LGTM 👍

[jezdez]

@webknjaz It would be great to follow patterns that we've established in other Jazzband repos, e.g. the pip cache dir should not be hardcoded here. Instead use pip cache dir to get it, like in contextlib2. Thank you!

[webknjaz]

@jezdez got it, thanks for the feedback!

I think I copied this from one of my projects where pip cache dir is not a thing because it's old so I didn't bother overcomplicating it there. But I see the value of having it identified dynamically.

[jezdez]

@jezdez got it, thanks for the feedback!

I think I copied this from one of my projects where pip cache dir is not a thing because it's old so I didn't bother overcomplicating it there. But I see the value of having it identified dynamically.

Gotcha, we switched to that CLI call only a bit ago when pip gained this subcommand, so no worries!

[webknjaz]

#1389

[hugovk]

pip can now be cached via actions/setup-python to save a lot of config:

https://github.com/actions/setup-python#caching-packages-dependencies

Please see #1571 for an alternative.