Gevent runtime uses python-level calls internally which might interfere
with current python state. For example if current python exception is
set, and e.g. go or makesema runtime call is made, the following
happens:

    golang/golang_test.py::test_pyx_runtime_vs_pyexc RuntimeError: abc

    The above exception was the direct cause of the following exception:

    SystemError: <class 'gevent.__semaphore.Semaphore'> returned a result with an error set
    Exception ignored in: 'golang.runtime._runtime_gevent._sema_alloc'
    SystemError: <class 'gevent.__semaphore.Semaphore'> returned a result with an error set
    terminate called after throwing an instance of 'golang::PanicError'
      what():  makesema: alloc failed
    Fatal Python error: Aborted

-> Fix all functions in the runtimes that work at Python level to
   save/restore Python-level exception on entry/exit.

This is mostly gevent runtime, but also a couple of non-posix fallbacks in
thread runtime.

The bug was there from day 1 of runtimes - from ce8152a2 (pyx api:
Provide sleep), f971a2a8 (pyx api: Provide go) and 69db91bf (libgolang:
Add internal semaphores).