python: Mark intentionally unused imports with noop statements.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

tools/lib/sanity_check.py

@@ -8,6 +8,9 @@ def check_venv(filename):
         import django
         import ujson
         import zulip
+        django
+        ujson
+        zulip
     except ImportError:
         print("You need to run %s inside a Zulip dev environment." % (filename,))
         user_id = os.getuid()

zerver/apps.py

@@ -22,6 +22,7 @@ class ZerverConfig(AppConfig):
         # running that code too early in Django's setup process, but
         # in any case, this is an intentionally unused import.
         import zerver.signals
+        zerver.signals
 
         if settings.POST_MIGRATION_CACHE_FLUSHING:
             post_migrate.connect(flush_cache, sender=self)

zproject/wsgi.py

@@ -28,6 +28,7 @@ django.setup()  # We need to call setup to load applications.
 # need to import zerver.models first before the middleware tries to import it.
 
 import zerver.models
+zerver.models
 
 # This application object is used by any WSGI server configured to use this
 # file. This includes Django's development server, if the WSGI_APPLICATION