Adhoc distribtest flow: https://g3c.corp.google.com/results/invocations/7a9a3047-95ec-4b14-9607-1a80d2f6532b

Resulting grpcio-1.31.0.dev0.tar.gz for the adhoc build https://pantheon.corp.google.com/storage/browser/grpc-testing-kokoro-prod/test_result_public%2Fprod%2Fgrpc%2Fcore%2Fmaster%2Flinux%2Fgrpc_build_packages%2F13059%2F20200721-041828%2Fgithub%2Fgrpc%2Fartifacts%2F still doesn't contain the .hpp files, looks like python is actively resisting adding them.
Maybe the graft command and recursive-include are clashing with each other?

I think I found the real reason why upb.hpp is not included in the python source wheel and it's really wicked :-)

I think graft third_party/upb works just fine, but then the *.hpp file gets stripped out of the tar.gz

I'm testing a fix right now.

Also looks like I'm not the only one who got bitten by the "garbage exclusion": #19891 (found that when searching though history of build_artifact_python.sh).
CC @veblush :-)

I verified locally that the file upb.hpp is not missing anymore from the sdist package.
(by running DOCKERHUB_ORGANIZATION=grpctesting tools/run_tests/task_runner.py -f python artifact manylinux2010 cp37-cp37m x64 and inspecting the grpcio-1.31.0.dev0.tar.gz under artifacts/ directory)

I started another adhoc distribtest flow to double check:
https://g3c.corp.google.com/results/invocations/d155a522-30a2-42fd-9223-d00577b5473a

Created a separate PR with a fix: #23565

All distribtests are passing: https://sponge.corp.google.com/target?id=eb9511e8-30f4-41ff-8847-76b105f84a30&target=grpc%2Fcore%2Fexperimental%2Fgrpc_distribtests_multiplatform&searchFor= (which also means that both #23552 and #23565 are good fixes).