extract_recipients: Enforce str as incoming type.

After removing internal_send_message() in a recent
commit, we now have only two callers for
extract_recipients, and they are both related
to our REQ mechanism that always passes strings
to converters.  (If there are default values,
REQ does not call the converters.)

We therefore make two changes:

    - use the more strict annotation of "str"
      for the `s` parameter

    - don't bother with the isinstance check

zerver/lib/actions.py

@@ -2028,18 +2028,13 @@ def already_sent_mirrored_message_id(message: Message) -> Optional[int]:
         return messages[0].id
     return None
 
-def extract_recipients(
-        s: Union[str, Iterable[str], Iterable[int]]
-) -> Union[List[str], List[int]]:
+def extract_recipients(s: str) -> Union[List[str], List[int]]:
     # We try to accept multiple incoming formats for recipients.
     # See test_extract_recipients() for examples of what we allow.
 
-    if isinstance(s, str):
-        try:
-            data = ujson.loads(s)
-        except (ValueError, TypeError):
-            data = s
-    else:
+    try:
+        data = ujson.loads(s)
+    except (ValueError, TypeError):
         data = s
 
     if isinstance(data, str):

zerver/tests/test_messages.py

@@ -652,7 +652,7 @@ class ExtractedRecipientsTest(TestCase):
             extract_recipients(s)
 
         # Empty list
-        self.assertEqual(extract_recipients([]), [])
+        self.assertEqual(extract_recipients('[]'), [])
 
         # Heterogeneous lists are not supported
         mixed = ujson.dumps(['eeshan@example.com', 3, 4])