lang/python*: Replace DISABLED_EXTENSIONS with Setup.local

Currently, lang/python38 and lang/python39 don't honor
DISABLED_EXTENSIONS because patch-issue20210 was removed when
lang/python38 was added to the ports tree. patch-issue20210 is still
present on lang/python36 and lang/python37.

Building with poudriere is not affected because builds are executed in a
clean environment.

Setup.local is the more canonical and recommended method for customizing
Python builds for shared extensions & third party libraries.

Support for a *disabled* marker in Setup files was introduced in Python
3.7, so backport this fix to it to keep consistency in the ports tree.

PR: 243358 [1]
PR: 243937 [2]
Reported by: ngie [1]
Reported by: jcfyecrayz@liamekaens.com [2]
Reported by: tuxillo (IRC) DPorts
Reviewed by: koobs (python, maintainer)
Approved by: koobs, dbaio (python, maintainer)
Differential Revision: https://reviews.freebsd.org/D31086

(cherry picked from commit a94d4b1005b1e93a27bcb9e4e794eeb13c991dd5)