Support __setattr__

[JukkaL]

See #520

[gvanrossum]

This is blocking some typeshed improvements, e.g. threading.locals and https://github.com/python/typeshed/blob/master/stdlib/2and3/argparse.pyi#L115 (the latter doesn't actually work!).

[gnprice]

Specifically, if I understand this right, the idea is to make it so that if a type has a __setattr__, we'll allow setting any attribute with a value of a type compatible with the __setattr__ parameter type.

[gvanrossum]

This is now the oldest high priority issue...