python: Clean up various if False blocks.

Most of these are now-unnecessary typing imports; some are just
improved comments for those with other mypy motivations.

zerver/lib/avatar.py

@@ -1,8 +1,5 @@
 from django.conf import settings
 
-if False:
-    from zerver.models import UserProfile
-
 from typing import Any, Dict, Optional
 
 from zerver.lib.avatar_hash import gravatar_hash, user_avatar_path_from_ids

zerver/lib/bot_lib.py

@@ -17,8 +17,7 @@ from zerver.lib.topic import get_topic_from_message_info
 
 import configparser
 
-if False:
-    from mypy_extensions import NoReturn
+from mypy_extensions import NoReturn
 from typing import Any, Optional, List, Dict
 from types import ModuleType
 

zerver/lib/cache.py

@@ -19,9 +19,9 @@ import os
 import hashlib
 
 if False:
-    from zerver.models import UserProfile, Realm, Message
     # These modules have to be imported for type annotations but
     # they cannot be imported at runtime due to cyclic dependency.
+    from zerver.models import UserProfile, Realm, Message
 
 ReturnT = TypeVar('ReturnT')  # Useful for matching return types via Callable[..., ReturnT]
 

zerver/lib/test_helpers.py

@@ -51,6 +51,7 @@ from zerver.models import (
 from zerver.lib.request import JsonableError
 
 if False:
+    # Avoid an import cycle; we only need these for type annotations.
     from zerver.lib.test_classes import ZulipTestCase, MigrationsTestCase
 
 import collections