ci: Remove Django setuptools pin

Revert #3371, which was needed to work around pypa/setuptools#4519 and allow our Django tests to run on Python 3.12. pypa/setuptools#4519 has been resolved upstream, so the workaround should no longer be needed.

Author: szokeasaurusrex

constraints.txt

@@ -646,7 +646,6 @@ setenv =
     OBJC_DISABLE_INITIALIZE_FORK_SAFETY=YES
     COVERAGE_FILE=.coverage-{envname}
     django: DJANGO_SETTINGS_MODULE=tests.integrations.django.myapp.settings
-    py3.12-django: PIP_CONSTRAINT=constraints.txt
 
     common: TESTPATH=tests
     gevent: TESTPATH=tests