Add versioning and package metadata for pydiffx.
Review Request #11749 — Created July 28, 2021 and submitted
This adds our standard versioning variables and functions for pydiffx,
defined in a
pydiffx._versionmodule and aliased in the top-level
Package infrastructure has been updated with some new metadata for
generating correct packages, and an
--all-pyversoption to run
packaging commands against all supported versions of Python.
Successfully built the package. Inspected the metadata for issues.