FIX: numpy 2.0 support, adding some pytest tests

[ken-lauer]

• numpy 2.0 was recently released and has broken a few codebases, including distgen
  • np.string_ -> np.bytes_
  • np.Inf -> np.inf
  • Full migration guide is here: https://numpy.org/devdocs/numpy_2_0_migration_guide.html#numpy-2-migration-guide
• Added some tests based on the regression test notebooks. I only converted two:
  • beam: This was easy to parametrize with pytest fixtures based on how nicely you had the notebook laid out.
  • dist: This was relatively easy to convert as most of the tests were standalone
• Added GitHub Actions test runners for the supported Python versions