I am getting ready to move almost everything under the upb/ directory up one
level to integrate upb better into its new location in the protobuf repo. This
change makes a few tweaks to prepare for that:
- Delete upb's LICENSE and CONTRIBUTING.md files since we already have similar
files at the top level.
- Rename `//python:python_version` so that it won't conflict later with
`//upb/python:python_version`.
- Move the contents of python/BUILD.bazel out to a Bazel macro to facilitate
merging that BUILD.bazel file with upb/python/BUILD.
PiperOrigin-RevId: 567119840
