www/py-dj52-django-stubs-ext: Add slave port of www/py-django-stubs-ext

sysutils/py-healthchecks port requires django 5.2 and
www/py-django-stubs-ext uses django 4.2 so a conflict will arise. The
slave port has been created to use DJANGO_VERSION with the specific
version of django that a port needs.

PR: 287486
Approved by: acm (mentor)
Approved by: maintainer timeout (2 weeks)