New port: editors/py-pynvim: Python client and plugin host for Neovim

Pynvim implements support for python plugins in Nvim. It also works as a
library for connecting to and scripting Nvim processes through its
msgpack-rpc API.

WWW: https://pynvim.readthedocs.io/en/latest/

Reviewed by: mat
Approved by: mat (mentor)
Differential Revision: https://reviews.freebsd.org/D19597