manage.py: Clean up imports a bit.

Import at fewer separate places in the code, and without duplication.

manage.py

@@ -10,23 +10,20 @@
 import scripts.lib.setup_path_on_import
 
 if __name__ == "__main__":
-    if 'posix' in os.name and os.geteuid() == 0:
-        from django.core.management.base import CommandError
-        raise CommandError("manage.py should not be run as root.  Use `su zulip` to drop root.")
     os.environ.setdefault("DJANGO_SETTINGS_MODULE", "zproject.settings")
-    os.environ.setdefault("PYTHONSTARTUP", os.path.join(BASE_DIR, "scripts/lib/pythonrc.py"))
-
     from django.conf import settings
+    from django.core.management import execute_from_command_line
     from django.core.management.base import CommandError
     from scripts.lib.zulip_tools import log_management_command
 
+    if 'posix' in os.name and os.geteuid() == 0:
+        raise CommandError("manage.py should not be run as root.  Use `su zulip` to drop root.")
+
     log_management_command(" ".join(sys.argv), settings.MANAGEMENT_LOG_PATH)
 
+    os.environ.setdefault("PYTHONSTARTUP", os.path.join(BASE_DIR, "scripts/lib/pythonrc.py"))
     if "--no-traceback" not in sys.argv and len(sys.argv) > 1:
         sys.argv.append("--traceback")
-
-    from django.core.management import execute_from_command_line
-
     try:
         execute_from_command_line(sys.argv)
     except CommandError as e: