fix python 3.7

Author: IAmTomahawkx

requirements.txt

@@ -1,2 +1,3 @@
 aiohttp>=3.6.0,<3.8.0
-iso8601
+iso8601
+typing-extensions

twitchio/ext/commands/core.py

@@ -27,7 +27,8 @@
 
 import itertools
 import copy
-from typing import Any, Union, Optional, Callable, Awaitable, Tuple, TYPE_CHECKING, Literal, List, Type, Set, TypeVar
+from typing import Any, Union, Optional, Callable, Awaitable, Tuple, TYPE_CHECKING, List, Type, Set, TypeVar
+from typing_extensions import Literal
 
 from twitchio.abcs import Messageable
 from .cooldowns import *