A subset of lib2to3 taken from Python 3.7.0b2.
Commit hash: 9c17e3a1987004b8bcfbe423953aad84493a7984

Reasons for forking:
- consistent handling of f-strings for users of Python < 3.6.2
- backport of BPO-33064 that fixes parsing files with trailing commas after
  *args and **kwargs
- backport of GH-6143 that restores the ability to reformat legacy usage of
  `async`
- support all types of string literals
- better ability to debug (better reprs)
- INDENT and DEDENT don't hold whitespace and comment prefixes
- ability to Cythonize