Use Tuple instead of tuple in type signature

sygnal/gcmpushkin.py

@@ -745,7 +745,7 @@ def _build_data(
  return data
 
 
-def truncate_str(input: str, max_bytes: int) -> tuple[str, bool]:
+def truncate_str(input: str, max_bytes: int) -> Tuple[str, bool]:
  """
  Truncate the given string. If the truncation would occur in the middle of a unicode
  character, that character will be removed entirely instead.