protobuf: add Python 3.7 compatibility

[benmwebb]

If built using --with-python, compilation fails with
Python 3.7 because the Python folks changed their C
API such that PyUnicode_AsUTF8AndSize() now returns
a const char* rather than a char*. Add a patch to
work around.

• Have you followed the guidelines for contributing?
• Have you checked that there aren't other open pull requests for the same formula update/change?
• Have you built your formula locally with brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• Does your build pass brew audit --strict <formula> (after doing brew install <formula>)?

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[ilovezfs]

Thanks for the PR @benmwebb. This needs to be submitted upstream as a PR and then we can use a patch do block in the formula to apply the upstream PR as a patch.

[benmwebb]

OK, I think I can do that. I was just following the example of OpenCV, which patches directly in the formula: 377ac4c. Figured that was the approved way to handle such issues.

[ilovezfs]

@benmwebb see #29707 :)

[benmwebb]

Upstream PR: protocolbuffers/protobuf#4862. I can update this PR once the protobuf folks merge that (or otherwise fix Python 3.7), if that works best for you guys.

[ilovezfs]

Just use "https://github.com/google/protobuf/pull/4862.patch?full_index=1"

[ilovezfs]

i.e. no need to wait

[benmwebb]

OK, PR updated to use the upstream patch.

[ilovezfs]

Thanks @benmwebb! Shipped.