Summary:
With `from __future__ import annotations`, all type hints become strings. The current type validator assumes that what is being checked is a type and not a string, so no types are being validated in these cases.

The fix is to use Python's typing.get_type_hints to evaluate the type hints at runtime

- [x] Added tests, if you've added code that should be tested
- [x] Ensured the test suite passes
- [x] Made sure your code lints
- [x] Completed the Contributor License Agreement ("CLA")

Pull Request resolved: facebook#333

Test Plan:
Imported from GitHub, without a `Test Plan:` line.

New unit tests were added to check that function parameters and return types were being interpreted correctly when annotated as a string (which is the case with from __future__ import annotations)

Reviewed By: deathowl

Differential Revision: D35778959

Pulled By: williamlw999-fb

fbshipit-source-id: 20ad9fbbb22c8189a9ba650729b99f5c59a24a16