This adds Python 3.9 and 3.10 typing syntax to files that import
attributions from __future___. Python 3.9 should be able to cope with
the 3.10 syntax, but Python 3.8 will no longer work.
On Windows/Mac, install the latest Python 3.9 version from python.org.
There are currently no orjson wheels for Python 3.10 on Windows/Mac,
which will break the build unless you have Rust installed separately.
On Linux, modern distros should have Python 3.9 available already. If
you're on an older distro, you'll need to build Python from source first.
Will allow importing the Protobuf without pulling in the rest of
the library. This is not a full PEP420 namespace, and the wheel still
bundles everything - it just makes things easier in a Bazel workspace.
I originally tried with PEP420, but it required more invasive changes,
and I ran into issues with mypy.
