build: Don't require the backports.zoneinfo package for newer python.

We do this so that we can build requirements files that could work to
`pip install` on both the old (3.8) and new (3.11) versions of python.

scripts/user_retirement/requirements/base.txt

@@ -10,8 +10,9 @@ attrs==23.2.0
     # via zeep
 backoff==2.2.1
     # via -r scripts/user_retirement/requirements/base.in
-backports-zoneinfo==0.2.1
+backports-zoneinfo==0.2.1 ; python_version < "3.9"
     # via
+    #   -c scripts/user_retirement/requirements/../../../requirements/constraints.txt
     #   django
     #   pendulum
 boto3==1.34.26

scripts/user_retirement/requirements/testing.txt

@@ -14,7 +14,7 @@ attrs==23.2.0
     #   zeep
 backoff==2.2.1
     # via -r scripts/user_retirement/requirements/base.txt
-backports-zoneinfo==0.2.1
+backports-zoneinfo==0.2.1 ; python_version < "3.9"
     # via
     #   -r scripts/user_retirement/requirements/base.txt
     #   django