lang/python*: Replace DISABLED_EXTENSIONS with Setup.local

[dbaio]

Summary

lang/python*: Replace DISABLED_EXTENSIONS with Setup.local

Currently, lang/python38 and lang/python39 don't honour
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
customising 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:	[e-mail] [2]
Reported by:	tuxillo (IRC) DPorts
Reviewed_by:    koobs (python, maintainer)
Approved by:    koobs, danilo (python, maintainer)
MFH:            2020Q3 (build bugfix)

Test Plan

• testport: OK (poudriere, lang/python??, $modules tested)

[koobs]

Change looks good. Can we confirm successful QA for the three supported modules, by forcing BUILD_DEPENDS on them so it can be tested in poudriere

[dbaio]

In D31086#699349, @koobs wrote:

Change looks good. Can we confirm successful QA for the three supported modules, by forcing BUILD_DEPENDS on them so it can be tested in poudriere

Thanks koobs... And yes, it's all good on my tests.
I'll push it.

[koobs]

@danilo 👍 just update the test plan section to mention those combinations that were tested