Reintroduce setup.py changes from #8280 erased by piper import #8902

[jtattermusch]

This fixes the broken python aarch64 linux tests (see https://source.cloud.google.com/results/invocations/861e900c-12ad-4998-9489-18f5cfbf76ee)

For some reason (bad merge?), the changes needed for crosscompiling aarch64 python wheels have been removed in piper import https://github.com/protocolbuffers/protobuf/pull/8902/files#diff-eb8b42d9346d0a5d371facf21a8bfa2d16fb49e213ae7c21f03863accebe0fcfL161.

This is actually the second time this happened (see #8667 and the fix #8746). What can we do to make sure this won't happen again?

[haberman]

Yes there does appear to be a bug in the tooling where setup.py changes aren't getting merged properly. We should take a look at at that.

I do wonder though, why wasn't this caught in Kokoro testing? Shouldn't there be a test verifying this behavior?

[jtattermusch]

There is a test job and the failure was caught by it: https://fusion.corp.google.com/projectanalysis/summary/KOKORO/prod:protobuf%2Fgithub%2Fmaster%2Fubuntu%2Fpython_aarch64%2Fcontinuous

the test doesn't run on PRs though (I didn't want to add a large number of new PR test jobs) - we can add those jobs to protobuf PRs if you think it's worthwhile.

[jtattermusch]

Btw it seems that the 3.17.x release has this snippet missing:

https://github.com/protocolbuffers/protobuf/blob/3.17.x/python/setup.py

while 3.16.x and 3.18.x have it:
https://github.com/protocolbuffers/protobuf/blob/3.16.x/python/setup.py
https://github.com/protocolbuffers/protobuf/blob/3.18.x/python/setup.py

[peteboothroyd]

@haberman @jtattermusch this has also affected this PR #8346 which has been reverted and lost.

[jtattermusch]

FTR I filed internal b/213024295 to track the piper import problem.