typing: Broaden type annotations for QuerySet compatibility.

To explain the rationale of this change, for example, there is
`get_user_activity_summary` which accepts either a `Collection[UserActivity]`,
where `QuerySet[T]` is not strictly `Sequence[T]` because its slicing behavior
is different from the `Protocol`, making `Collection` necessary.

Similarily, we should have `Iterable[T]` instead of `List[T]` so that
`QuerySet[T]` will also be an acceptable subtype, or `Sequence[T]` when we
also expect it to be indexed.

Signed-off-by: Zixuan James Li <p359101898@gmail.com>

zerver/lib/digest.py

@@ -2,7 +2,7 @@ import datetime
 import heapq
 import logging
 from collections import defaultdict
-from typing import Any, Dict, List, Set, Tuple
+from typing import Any, Collection, Dict, List, Set, Tuple
 
 from django.conf import settings
 from django.db import transaction
@@ -276,10 +276,12 @@ def get_slim_stream_map(stream_ids: Set[int]) -> Dict[int, Stream]:
     return {stream.id: stream for stream in streams}
 
 
-def bulk_get_digest_context(users: List[UserProfile], cutoff: float) -> Dict[int, Dict[str, Any]]:
+def bulk_get_digest_context(
+    users: Collection[UserProfile], cutoff: float
+) -> Dict[int, Dict[str, Any]]:
     # We expect a non-empty list of users all from the same realm.
     assert users
-    realm = users[0].realm
+    realm = next(iter(users)).realm
     for user in users:
         assert user.realm_id == realm.id