Fix new flake8 errors (#7489)

This is a cherry-pick of 1a1da60ad2c9172fe487cd38a164b39df60f4cb5 (#7470)
to the release-v1.13.0 branch.

1 files changed, 3 insertions, 2 deletions

diff --git a/synapse/app/_base.py b/synapse/app/_base.py

index 628292b890..dedff81af3 100644
--- a/synapse/app/_base.py
+++ b/synapse/app/_base.py
@@ -22,6 +22,7 @@ import sys
 import traceback
 
 from daemonize import Daemonize
+from typing_extensions import NoReturn
 
 from twisted.internet import defer, error, reactor
 from twisted.protocols.tls import TLSMemoryBIOFactory
@@ -139,9 +140,9 @@ def start_reactor(
             run()
 
 
-def quit_with_error(error_string):
+def quit_with_error(error_string: str) -> NoReturn:
     message_lines = error_string.split("\n")
-    line_length = max(len(l) for l in message_lines if len(l) < 80) + 2
+    line_length = max(len(line) for line in message_lines if len(line) < 80) + 2
     sys.stderr.write("*" * line_length + "\n")
     for line in message_lines:
         sys.stderr.write(" %s\n" % (line.rstrip(),))