Add process initializer (from Python 3.7) to complete issue 21423 implementation

[fahhem]

Also updates the docs for thread and process pools.

Unfortunately, the ProcessPoolExecutor is vastly differently implemented than upstream Python 3, so I couldn't use stick to the upstream PR as much as for thread pools: https://github.com/python/cpython/pull/4241/files

Is there any interest in wholesale copying the Python 3.7 process pool executor instead of redoing the bpo's out of order?

[s0undt3ch]

After merge what would be the expected time frame to release it?

[agronholm]

Also updates the docs for thread and process pools.

But, there should also be new tests. Surely Python 3 had some added when this feature was introduced?

Is there any interest in wholesale copying the Python 3.7 process pool executor instead of redoing the bpo's out of order?

IIRC the new process pool executor relies on Python 3 features. Otherwise would've been nice because process pools are broken beyond repair on Python 2.

After merge what would be the expected time frame to release it?

Within a day or two. It should be noted that since Python 2 has now reached its End Of Life, I will not be adding any more features to this library beyond this one. If somebody else wants to take over, that is negotiable.