build: Explicitly restrict backports.zoneinfo to python<3.9

For somereason whene pip-compile is compiling requiremnets it's not
pulling this environment marker from the upstream dependency so we need
to add this depedency of django explicitly for now so that we can
restrict it to python<3.9 and have that show up in our requirements
files.

requirements/django.in

@@ -8,3 +8,7 @@
 Django
 openedx-django-pyfs>=1.0.5
 lazy
+
+# Needed until we drop testing on Python 3.8 so that
+# we don't try to install this on newer versions of python.
+backports.zoneinfo;python_version<"3.9"