Feature request: Python3 support

[carlpaten]

Ubuntu hasn't shipped Vim with +python since 16.04. It'd be nice to have +python3 support in coquille.

[carlpaten]

I've created a partial port at lilred/coquille. Pull requests welcome.

Known issues:

• Vim hangs on moderately large multi-file programs. (Seen in the Induction.v chapter of Software Foundations.)

[bluelightning32]

Added python 3 support in #66