WIP: More transformers

[luk3yx]

This PR adds more transformers and fixes some mypy errors. Note that this probably adds too many transformers (in particular the targets below 2.7), and I have marked it as a draft because of this. This also interprets and transpiles f-strings differently, fixing #47.

Transformers added

Target 3.7:

• Certain walrus operators - This is rather hit and miss, and some walrus operators currently only work on CPython and only if the variable has already been defined in the same scope.
• Positional only parameters
• Self-documenting f-string expressions (works automatically)

Target 3.5:

• Asynchronous generators

Target 2.7:

• Keyword only arguments
• The nonlocal statement, provided you don't try and check for variables used with nonlocal in locals().
• A __nonzero__ alias for any __bool__ methods.
• Non-ASCII identifiers. Non-ASCII identifiers are mangled currently mangled in a similar way to Hy.

Target 2.6:

• Class decorators
• Dict comprehension
• Set literals

Target 2.5:

• six.print_() instead of print().
• six.advance_iterator() instead of next().
• except as (note that this breaks compatibility with Python 3.0+).
• Keyword arguments after *args.
• An itertools.zip_longest backport.