With some delay, this is a PR for
#32564 (and previously
#31791).

I looked into adding a regular `py_test` for this change [as
suggested](#31791 (comment))
but I am not aware of any effect that the presence of a .pyi stub file
would have at runtime and where some sort of type-checking in a .py
script would be affected. Stub files are only for use by type checkers &
IDE's. I mean, something like this would work:

```
import helloworld_pb2
py_file = helloworld_pb2.__file__ 
pyi_file = py_file + 'i’
self.assertTrue(os.path.exists(pyi_file))
```

But that seems really hacky to me. Instead I created a simple rule test
for `py_proto_library` with Bazel Skylib which tests the declared
outputs for an example `py_proto_library` target. Indirectly, this also
tests that the declared output files are actually generated. Please let
me know if this is sufficient.

With some delay, this is a PR for
grpc#32564 (and previously
grpc#31791).

I looked into adding a regular `py_test` for this change [as
suggested](grpc#31791 (comment))
but I am not aware of any effect that the presence of a .pyi stub file
would have at runtime and where some sort of type-checking in a .py
script would be affected. Stub files are only for use by type checkers &
IDE's. I mean, something like this would work:

```
import helloworld_pb2
py_file = helloworld_pb2.__file__ 
pyi_file = py_file + 'i’
self.assertTrue(os.path.exists(pyi_file))
```

But that seems really hacky to me. Instead I created a simple rule test
for `py_proto_library` with Bazel Skylib which tests the declared
outputs for an example `py_proto_library` target. Indirectly, this also
tests that the declared output files are actually generated. Please let
me know if this is sufficient.