After mypy [started hiding](https://github.com/python/mypy/pull/3706) imported names in stubs unless `from ... import ...` is used, we found an error with stubs of ast module.
It looks like ast module should re-export everything in `_ast` and according to PEP 484, it can do that by doing `from _ast import *`, so this is what this PR does.
python/mypy#2380 showed a discrepancy between object and FunctionType in stdlib2. The first defined __doc__ to be str, the second Optional[str]. As FunctionType depends on object, this is no longer valid.
As suggested by @gvanrossum in python/mypy#2380, all __doc__ should be considered Optional.
(Final verdict was just to remove most __doc__ attributes since it's inherited from object.)
