As suggested by https://github.com/jayvdb run manifest checker and oops,
it finds that we forgot to include pyproject.toml for testprog/golang_pyx_user:

    missing from sdist:
      golang/pyx/testprog/golang_pyx_user/pyproject.toml
    suggested MANIFEST.in rules:
      recursive-include golang *.toml

-> Fix MANIFEST.in in generic way to include golang/*/*.toml

The bug was not affecting pygolang usage - only golang.pyx.build tests were
failing for sdist installed pygolang.