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
- better ability to debug (better reprs for starters)
- ability to Cythonize