Add a GitHub Actions workflow for Python releases.
Add a GitHub Actions workflow for Python releases.

It will be triggered manually: it requires a build number to be specified
via the GitHub Actions UI so that bumping the version number isn't needed
just to rebuild with the latest RE2 and/or Abseil code.

For now, nothing is uploaded to PyPI. I want to inspect the logs first...

Change-Id: Icc86423e957f927e51de28f735549ebd76653428
Reviewed-on: https://code-review.googlesource.com/c/re2/+/60612
Reviewed-by: Alex Chernyakhovsky <7d8f1034c38249e22bb157c14849fb2873ab27f4@google.com>
Reviewed-by: Paul Wankadia <62d4e38bb1380f055ec4b899baa75e52af09c5f7@google.com>

google/re2,google/re2,google/re2,google/re2,google/re2