Switch Literal import based on Python version.

pymispgalaxies/api.py

@@ -9,6 +9,10 @@ from collections.abc import Mapping
 from glob import glob
 import re
 from typing import List, Dict, Optional, Any, Tuple, Iterator, overload, Union
+
+if sys.version_info >= (3, 8):
+    from typing import Literal
+else:
     from typing_extensions import Literal
 
 try: