lang/python38: Deprecate/Set to expire

* Python 3.8 will reach its End-of-Life on 2024-10. Set the expiration
  date accordingly.

Approved by:	lwhsu (python, maintainer)
Differential Revision:	https://reviews.freebsd.org/D43345

lang/python38/Makefile

@@ -13,6 +13,9 @@ WWW=		https://www.python.org/
 
 LICENSE=	PSFL
 
+DEPRECATED=	Upgrade to a newer Python version. 3.8 is in maintenance status and gets security fixes only. End-of-Life: 2024-10. See https://devguide.python.org/versions/
+EXPIRATION_DATE=2024-10-31
+
 LIB_DEPENDS=	libffi.so:devel/libffi
 
 USES=		cpe ncurses pathfix pkgconfig python:${PYTHON_DISTVERSION:R},env readline \