pygil_ensure() was taking pyexitedMu and then GIL, which can deadlock
via AB BA scenario if pygil_ensure() is also called in another thread
with GIL already held.

-> Fix it by always taking GIL with pyexitedMu released.

Without the fix the test added always hangs with thread backend, e.g.
like this:

    golang/pyx/runtime_test.py::test_pyx_pyfunc_vs_gil_deadlock
    STUCK
    terminate called after throwing an instance of 'golang::PanicError'
      what():  STUCK
    Fatal Python error: Aborted

The bug was introduced in b073f6df (time: Move/Port timers to C++/Pyx nogil).