Move manage.py entrypoint to its own function

manage.py

@@ -2,14 +2,19 @@
 import os
 import sys
 
-if __name__ == "__main__":
+try:
+    from django.core.management import execute_from_command_line
+except ImportError as exc:
+    raise ImportError(
+        "Couldn't import Django. "
+        "Run `poetry shell` to activate a virtual environment first."
+    ) from exc
+
+
+def main():
     os.environ.setdefault("DJANGO_SETTINGS_MODULE", "tests.settings")
-    try:
-        from django.core.management import execute_from_command_line
-    except ImportError as exc:
-        raise ImportError(
-            "Couldn't import Django. Are you sure it's installed and "
-            "available on your PYTHONPATH environment variable? Did you "
-            "forget to activate a virtual environment?"
-        ) from exc
     execute_from_command_line(sys.argv)
+
+
+if __name__ == "__main__":
+    main()