Determine if we should have a presubmit for PyPy

[parthea]

As part of the repo consolidation project, the PyPy3.10 presubmit was removed from proto-plus. The reason is that 3.10 is no longer supported on the PyPy release page: https://downloads.python.org/pypy/

See googleapis/proto-plus-python@8be46a0 from googleapis/proto-plus-python#507 where we needed a presubmit for PyPy to catch googleapis/proto-plus-python#506.

As of writing only Python 3.11 is supported in PyPy. If newer runtimes are supported we may want to add presubmits for PyPy in gapic-generator-python and downstream in google-cloud-python.

Monitor https://downloads.python.org/pypy/ for new releases

[daniel-sanche]

AFAIK we don't formally support pypy? Assuming that's the case, we can probably just drop these tests

grpc doesn't support pypy, so I think the use-case for our libraries would be limited