Allow passing lambdas as init arguments in multiprocessing

[Sterbic]

This will enable passing lambda expressions as init functions:

import signal
from multiprocessing import Manager

m = Manager()
m.start(lambda: signal.signal(signal.SIGINT, signal.SIG_IGN))

test.py:6: error: Argument 1 to "start" of "BaseManager" has incompatible type Callable[[], Union[Callable[[Signals, FrameType], None], int, Handlers]]; expected Callable[..., None]