Use full Python version for GitHub Actions cache key

[micahellison]

We keep running into stale cache issues when this happens:

• Someone opens a PR
• Python releases a new patch version (like from 3.9.7 to 3.9.8)
• The next time tests run on that PR, they crash because they're have references to the older patch version

This should fix the issue by grabbing the full python --version output such as "Python 3.9.8" and using that as part of the cache key.

Checklist

• I have read the contributing doc.
• I have included a link to the relevant issue number.
• I have checked to ensure there aren't other open pull requests
  for the same issue.
• I have written new tests for these changes, as needed.

[wren]

lgtm 🧨