[WIP/RFC] multiprocessing: proxy: keep _manager after forking

[blueyed]

Fixes iterating over dicts:

import multiprocessing as mp

def run(d):
    for k in d:
        print(k)

manager = mp.Manager()

d = manager.dict({1: 2, 3: 4})

process = mp.Process(target=run, args=(d,))
process.start()
process.join()

With this fix:

_callmethod __iter__ #PROXY (('__next__', 'send', 'throw', 'close'), Token(typeid='Iterator', address='/tmp/pymp-wfn3r3nd/listener-ze4ow_dl', id='7f12414ca130'))
_callmethod __next__ #RETURN 1
1
_callmethod __next__ #RETURN 3
3
_callmethod __next__ #ERROR

Without:

_callmethod __iter__ #PROXY (('__next__', 'send', 'throw', 'close'), Token(typeid='Iterator', address='/tmp/pymp-uppzhf9p/listener-mvmwwxma', id='7f5b7b42da90'))
Process Process-2:
Traceback (most recent call last):
  File "…/pyenv/3.8.0/lib/python3.8/multiprocessing/process.py", line 313, in _bootstrap
    self.run()
  File "…/pyenv/3.8.0/lib/python3.8/multiprocessing/process.py", line 108, in run
    self._target(*self._args, **self._kwargs)
  File "t-mp.py", line 5, in run
    for k in d:
  File "<string>", line 2, in __iter__
  File "…/pyenv/3.8.0/lib/python3.8/multiprocessing/managers.py", line 842, in _callmethod
    proxytype = self._manager._registry[token.typeid][-1]
AttributeError: 'NoneType' object has no attribute '_registry'

The line is from when multiprocessing was added in e711caf, so I am
probably missing something here? This PR is meant to get tests run with
it, and of course for any feedback already.

[blueyed]

/cc @benjaminp

[invisibleroads]

@benjaminp Do you know why self._manager is set to None in _after_fork?

The above-referenced issue prevents dictionary syntax like list(d), set(d), for x in d when using manager.dict()

However, list(d.keys()), set(d.keys()), for x in d.keys() work as noted in https://stackoverflow.com/a/70886363