Manually manage the GIL with Cython

[alxmrs]

I suspect that the performance of the ThreadPoolExecutor would substantially increase if we strategically placed cython with nogil calls.

• https://cython.readthedocs.io/en/latest/src/userguide/parallelism.html
• https://stackoverflow.com/questions/49047255/cython-nogil-with-threadpoolexecutor-not-giving-speedups
• https://stackoverflow.com/questions/56537989/usage-of-threadpoolexecutor-in-conjunction-with-cythons-nogil

There are drawbacks to process based parallelism; namely, there is a high overhead and IPC latency compared to multithreading.