Add a release workflow for PyPI in CI (Github Actions)

[wren]

More work on #1060 (getting close to done!)

• Support ci skip tag on commits to avoid build dupes
• Add smarter path detection so we don't spam tons of tests
• Allow steps to cancel if previous steps were cancelled (don't always
  run)
• Separate workflows to be more modular
• Update release workflow to do a few more things
• Add helpful messages to workflows
• Be more strict in version checking (now with added regex)
• Make changelog smarter about when to trigger
• Add some functionality for changelog to handle releases and
  prereleases separately
• Better error handling
• Split up the version validation and the release to make way for more
  releases

Checklist

• I have read the contributing doc.
• I have included a link to the relevant issue number.
• I have tested this code locally.
• 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.
• All tests pass.

[wren]

When this gets merged, the last thing for #1060 will be the homebrew stuff.

[github-actions]

Unit Test Results

  16 files  ±0    16 suites  ±0   10s ⏱️ +3s
288 tests ±0  288 ✔️ ±0  0 💤 ±0  0 ❌ ±0 
289 runs  ±0  289 ✔️ ±0  0 💤 ±0  0 ❌ ±0 

results for commit 6c06ed4 ± comparison against base commit e3aa088

[micahellison]

🚢