Fix new flake8 errors (#7489)

This is a cherry-pick of 1a1da60ad2 (#7470)
to the release-v1.13.0 branch.

synapse/push/mailer.py

@@ -19,6 +19,7 @@ import logging
 import time
 from email.mime.multipart import MIMEMultipart
 from email.mime.text import MIMEText
+from typing import Iterable, List, TypeVar
 
 from six.moves import urllib
 
@@ -41,6 +42,8 @@ from synapse.visibility import filter_events_for_client
 
 logger = logging.getLogger(__name__)
 
+T = TypeVar("T")
+
 
 MESSAGE_FROM_PERSON_IN_ROOM = (
     "You have a message on %(app)s from %(person)s in the %(room)s room..."
@@ -638,10 +641,10 @@ def safe_text(raw_text):
     )
 
 
-def deduped_ordered_list(l):
+def deduped_ordered_list(it: Iterable[T]) -> List[T]:
     seen = set()
     ret = []
-    for item in l:
+    for item in it:
         if item not in seen:
             seen.add(item)
             ret.append(item)