Add SynchronizedBase to __all__

Recently I was writing some heavily-parallel code,
and I tried to type-check with `mypy`. I (naively)
thought that `multiprocessing.Value` was a class
and was very confused when `mypy` threw an error
on that line. I dropped into the interpreter and
tried to use the `help` function to find the
return type of `Value` - and failed. I dropped
into the code and found that the type returned
from the `Value` function was not exported from
the module.

I want types in Python to be _awesome_. I think
`mypy` is a step in the right direction for that.
This change is me doing my part to expose more
types so `mypy` can become as good as it can be.

distortedsignal<distortedsignal@gmail.com>

Author: distortedsignal

Lib/multiprocessing/sharedctypes.py

@@ -16,7 +16,7 @@
 from .context import reduction, assert_spawning
 _ForkingPickler = reduction.ForkingPickler
 
-__all__ = ['RawValue', 'RawArray', 'Value', 'Array', 'copy', 'synchronized']
+__all__ = ['RawValue', 'RawArray', 'Value', 'Array', 'copy', 'synchronized', 'SynchronizedBase']
 
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