This gets imported from `_ast`, so the definition was
redundant. This causes problems with the new mypy semantic
analyzer, as it will flag the redefinition as an error.
When we import typeshed internally at Dropbox, somehow the fact that
these files are all stubs gets lost (it's a long story...). This
causes errors like this:
.../stdlib/2/ast.pyi:6: error: Name 'typing' already defined (by an import)
The quickest way around this is to rename the import to _typing.
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 As FunctionType depends on object, this is no longer valid.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.)
