Fix autogenerated protobuf stub's nonexistent import

[gvanrossum]

My PR (#4814) is failing tests in a part I didn't touch:

running mypy --python-version 3.9 --strict-optional --no-site-packages --show-traceback --no-implicit-optional --disallow-any-generics --disallow-subclassing-any --platform linux # with 1042 files
third_party/2and3/google/protobuf/util/json_format_proto3_pb2.pyi:45: error: Cannot find implementation or library stub for module named 'google.protobuf.unittest_pb2'
third_party/2and3/google/protobuf/util/json_format_proto3_pb2.pyi:45: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
Found 1 error in 1 file (checked 1042 source files)

(and repeated for other versions). The same seems to be happening for other recent PRs, e.g. #4813 and #4811.

[hauntsaninja]

Did some investigating, it looks like the trigger for this was syncing typeshed: python/mypy#9794
The error looks legitimate, the file was removed in https://github.com/python/typeshed/pull/4785/files#diff-328c07f017a31da756c9b78484dbeab4ec7757a29092a2ba2c9afe2d4bc63a50 but continues to be imported. I assume mypy managed to continue to find the file in its bundled typeshed, which is a little weird (maybe mypy_test should pass in --custom-typeshed-dir).

I'm not sure how to fix, since the file is autogenerated, so cc @nipunn1313

[hauntsaninja]

(In case Nipunn doesn't see this soon, just removing third_party/2and3/google/protobuf/util/json_format_proto3_pb2.pyi entirely seems to be enough to get things green in a pinch)

[srittau]

I have replaced the missing import with Any for now. This should work, but of course still needs a proper fix.