lang/python36: Deprecate and set to expire

Python 3.6 will reach its End-of-Life at 23rd December 2021.

Set the deprecation note and expiration date accordingly.

Approved by:	koobs (python, maintainer)
MFH:		No (not neccessary, 3+ months from now should be OK)
Differential Revision:	https://reviews.freebsd.org/D31783