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/webhooks/opbeat/view.py

@@ -9,7 +9,7 @@ from zerver.lib.response import json_success
 from zerver.lib.webhooks.common import check_send_webhook_message
 from zerver.models import UserProfile
 
-subject_types = {
+subject_types: Dict[str, List[List[str]]] = {
     'app': [  # Object type name
         ['name'],  # Title
         ['html_url'],  # Automatically put into title
@@ -37,7 +37,7 @@ subject_types = {
         ['in {}', 'extra/pathname'],
         ['!message']
     ]
-}  # type: Dict[str, List[List[str]]]
+}
 
 
 def get_value(_obj: Dict[str, Any], key: str) -> str:
@@ -56,7 +56,7 @@ def format_object(
 ) -> str:
     if subject_type not in subject_types.keys():
         return message
-    keys = subject_types[subject_type][1:]  # type: List[List[str]]
+    keys: List[List[str]] = subject_types[subject_type][1:]
     title = subject_types[subject_type][0]
     if title[0] != '':
         title_str = ''
@@ -65,7 +65,7 @@ def format_object(
         else:
             title_str = obj[title[0]]
         if obj['html_url'] is not None:
-            url = obj['html_url']  # type: str
+            url: str = obj['html_url']
             if 'opbeat.com' not in url:
                 url = 'https://opbeat.com/' + url.lstrip('/')
             message += '\n**[{}]({})**'.format(title_str, url)