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

lang/python36/Makefile

@@ -18,6 +18,9 @@ COMMENT=	Interpreted object-oriented programming language
 
 LICENSE=	PSFL
 
+DEPRECATED=	Upgrade to a newer Python version. 3.6 is in maintenance status and gets security fixes only. End-of-Life: 2021-12-23. See https://devguide.python.org/#status-of-python-branches
+EXPIRATION_DATE=	2021-12-31
+
 USES=		cpe ncurses pathfix pkgconfig readline shebangfix ssl tar:xz
 PATHFIX_MAKEFILEIN=	Makefile.pre.in
 USE_LDCONFIG=	yes