Add support for always tokenizing async/await as keywords

Fixes #593

I looked into this bug with @ambv and @carljm, and we reached the
conclusion was that it's not possible for the tokenizer to determine if
async/await is a keyword inside all possible generators without breaking
the grammar for older versions of Python.

Instead, we introduce a new tokenizer mode for Python 3.7+ that will
cause all async/await instances to get parsed as a reserved keyword,
which should fix async/await inside generators.