Add difflib stubs

This pull request adds stubs for the difflib module in the standard library.

Some notes:

- Right now this causes the tests to fail with the following:

  ```
  In module imported in _test_stubs.py, line 21:
  stubs/3.2/difflib.py, line 6: Invalid type "difflib._T"
  ```

  I'm not sure what that means or how to fix it. (Do named tuples not support polymorphic types?) Guidance would be appreciated.

- It would've been nice to be able to define a type alias for the rather wordy opcode type (`Tuple[str, int, int, int, int]`). When I tried that, I got other errors I didn't quite follow.
- The stubs are identical between with 2.7 and 3.2. Is there a way to define them without the code duplication?