There are even newer versions available (4.*), but I had found a bug
in it which prevented it from working with mypy-protobuf. For now,
sticking to just 3.20.1
I added stubtest testing for generated files within mypy-protobuf.
There are many ways in which the protobuf code is doing *weird*
things and we want the stubs to look a bit different. They're enumerated
in the `stubtest_allowlist.txt` of mypy-protobuf, so squashing them here
so that this one can focus on the non-generated files.
