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