workflows: Update CI action versions

[EliahKagan]

This updates versions of GitHub Actions, except in the stale.yml workflow. The upgraded actions in other workflows are checkout, setup-python, and gh-action-pypi-publish.

The checkout and setup-python actions were upgraded across major versions. Although it is possible for major version updates to introduce incompatibilities, I believe the advantages decisively outweigh the disadvantages, because:

• The syntax the workflows are using remains supported.
• They were upgraded in some places already, just not all.
• As noted in warning annotations shown on this project's workflow runs, old Node 12 actions may break around 27 September when GitHub Actions removes Node 12 support and runs all actions on Node 16.
• There may be a benefit to bugfixes in the newer versions.

For gh-action-pypi-publish, the minor version upgrade includes security updates to internal dependencies. This happened in 1.8.9.

I have not touched stale.yml here, even though the action it uses is specified to use a very old version, because that workflow is currently disabled (which appears to have been done automatically at some point during a lull in activity when no repository activity was detected for 60 days). Modifying it might re-enable it, which might not be desired.

[cwacek]

Disabling stale was actually intentional. It was marking things stale that weren't actually wrong, due to either a lack of time on my part or significant complexity.

[EliahKagan]

That makes sense. Would it be reasonable to remove the stale workflow?

[EliahKagan]

I've opened #256 to propose removal of the stale workflow.

(If you would prefer not to do that, then #256 can of course simply be closed without merging.)