I reported the bug (see 69db91bf "libgolang: Add internal
semaphores") to CPython and PyPy upstreams. PyPy people already fixed it
and the fix, if I understood correctly, should be available as part of
PyPy 7.2 . Let's hope that CPython 2.7 will be also fixed.

- https://bugs.python.org/issue38106
    -> https://github.com/python/cpython/pull/16047
- https://bitbucket.org/pypy/pypy/issues/3072

Added Python-level test triggers the bug with high probability.
Dedicated C-level Semaphore-only test is pending.