python: Convert assignment type annotations to Python 3.6 style.

This commit was split by tabbott; this piece covers the vast majority
of files in Zulip, but excludes scripts/, tools/, and puppet/ to help
ensure we at least show the right error messages for Xenial systems.

We can likely further refine the remaining pieces with some testing.

Generated by com2ann, with whitespace fixes and various manual fixes
for runtime issues:

-    invoiced_through: Optional[LicenseLedger] = models.ForeignKey(
+    invoiced_through: Optional["LicenseLedger"] = models.ForeignKey(

-_apns_client: Optional[APNsClient] = None
+_apns_client: Optional["APNsClient"] = None

-    notifications_stream: Optional[Stream] = models.ForeignKey('Stream', related_name='+', null=True, blank=True, on_delete=CASCADE)
-    signup_notifications_stream: Optional[Stream] = models.ForeignKey('Stream', related_name='+', null=True, blank=True, on_delete=CASCADE)
+    notifications_stream: Optional["Stream"] = models.ForeignKey('Stream', related_name='+', null=True, blank=True, on_delete=CASCADE)
+    signup_notifications_stream: Optional["Stream"] = models.ForeignKey('Stream', related_name='+', null=True, blank=True, on_delete=CASCADE)

-    author: Optional[UserProfile] = models.ForeignKey('UserProfile', blank=True, null=True, on_delete=CASCADE)
+    author: Optional["UserProfile"] = models.ForeignKey('UserProfile', blank=True, null=True, on_delete=CASCADE)

-    bot_owner: Optional[UserProfile] = models.ForeignKey('self', null=True, on_delete=models.SET_NULL)
+    bot_owner: Optional["UserProfile"] = models.ForeignKey('self', null=True, on_delete=models.SET_NULL)

-    default_sending_stream: Optional[Stream] = models.ForeignKey('zerver.Stream', null=True, related_name='+', on_delete=CASCADE)
-    default_events_register_stream: Optional[Stream] = models.ForeignKey('zerver.Stream', null=True, related_name='+', on_delete=CASCADE)
+    default_sending_stream: Optional["Stream"] = models.ForeignKey('zerver.Stream', null=True, related_name='+', on_delete=CASCADE)
+    default_events_register_stream: Optional["Stream"] = models.ForeignKey('zerver.Stream', null=True, related_name='+', on_delete=CASCADE)

-descriptors_by_handler_id: Dict[int, ClientDescriptor] = {}
+descriptors_by_handler_id: Dict[int, "ClientDescriptor"] = {}

-worker_classes: Dict[str, Type[QueueProcessingWorker]] = {}
-queues: Dict[str, Dict[str, Type[QueueProcessingWorker]]] = {}
+worker_classes: Dict[str, Type["QueueProcessingWorker"]] = {}
+queues: Dict[str, Dict[str, Type["QueueProcessingWorker"]]] = {}

-AUTH_LDAP_REVERSE_EMAIL_SEARCH: Optional[LDAPSearch] = None
+AUTH_LDAP_REVERSE_EMAIL_SEARCH: Optional["LDAPSearch"] = None

Signed-off-by: Anders Kaseorg <anders@zulipchat.com>

zerver/lib/hotspots.py

@@ -7,7 +7,7 @@ from zerver.models import UserProfile, UserHotspot
 
 from typing import List, Dict
 
-ALL_HOTSPOTS = {
+ALL_HOTSPOTS: Dict[str, Dict[str, str]] = {
     'intro_reply': {
         'title': _('Reply to a message'),
         'description': _('Click anywhere on a message to reply.'),
@@ -33,7 +33,7 @@ ALL_HOTSPOTS = {
         'description': _('Click here to start a new conversation. Pick a topic '
                          '(2-3 words is best), and give it a go!'),
     },
-}  # type: Dict[str, Dict[str, str]]
+}
 
 def get_next_hotspots(user: UserProfile) -> List[Dict[str, object]]:
     # For manual testing, it can be convenient to set