Follow PEP 484 - no implicit optionals

freqtrade/misc.py

@@ -6,7 +6,7 @@ import logging
 import re
 from datetime import datetime
 from pathlib import Path
-from typing import Any, Dict, Iterator, List, Mapping, Union
+from typing import Any, Dict, Iterator, List, Mapping, Optional, Union
 from typing.io import IO
 from urllib.parse import urlparse
 
@@ -205,7 +205,7 @@ def safe_value_fallback2(dict1: dictMap, dict2: dictMap, key1: str, key2: str, d
     return default_value
 
 
-def plural(num: float, singular: str, plural: str = None) -> str:
+def plural(num: float, singular: str, plural: Optional[str] = None) -> str:
     return singular if (num == 1 or num == -1) else plural or singular + 's'