Related to #144

We currently patch included packages to replace threading by dummy_threading imports. Also multiprocessing module is removed (and so fails to import), requiring us to patch any package that uses it.

In this PR,

• I remove all patches that replace threading by dummy_threading. Using dummy_threading will lead to deadlocks when thread communication is expected, while objects like threading.RLlock, threading.Lock are actually working fine. When threading.Thread(..).start() is called it will produce a "can't start new thread" exception which I think is a reasonable message in the context of pyodide.
  If the use of dummy_threading is desired it can be monkeypatched globally at run time cf Handling unsupported CPython stdlib modules #144 (comment)
• The multiprocessing module is no longer removed, so it will import without error. Surprisingly multiprocessing.cpu_count() returns 16 (maybe that is hard-coded in emscripten?). Starting a new Process will raise a Resource temporarily unavailable exception, while calling os.fork() which I think is also reasonable.

Overall the goal is to noticeably reduce the amount of patches we have to maintain, and also support more packages from PyPi with micropip without requiring patches.

I added tests to checks the above mentioned behavior. This also adds a few test improvements with a more systematic use of pytest.raises when needed.