Supporting feature_version=...

[gvanrossum]

Split off #95 -- this is a separate task.

Passing feature_version=(3, N) (or feature_version=N) to ast.parse() enables a few changes in the tokenizer and ast.c to support older Python versions. The primary motivation is to handle the three different stages of support for the async and await keywords, which is required by mypy.

Most of the logic is in the tokenizer, but we'll have to check ast.c for other tests for feature_version (e.g. there's also a test for @, the matrix multiply operator).

[lysnikolaou]

There are the following tests for feature_version in ast.c:

1. @ operator invalid in < 3.5
2. Async functions invalid in < 3.5
3. Async comprehensions invalid in < 3.6
4. Underscores in numeric literals invalid in < 3.6
5. Await expression invalid in < 3.5
6. Variable annotations invalid in < 3.6
7. Async for-loops invalid in < 3.5
8. Async with-statements invalid in < 3.5
9. F-strings invalid in < 3.6

I can take this up, but I'd prefer to do it after python#19780 is merged.

[gvanrossum]

Sounds great!