Do not require typing_extensions with Python 3.8+ (openai#154)

typing_extensions are only used for Literal which is available in the
standard library since Python 3.8

Author: paper42

openai/api_requestor.py

@@ -7,7 +7,12 @@
 from urllib.parse import urlencode, urlsplit, urlunsplit
 
 import requests
-from typing_extensions import Literal
+
+# Literal is available from Python 3.8
+try:
+    from typing import Literal
+except ImportError:
+    from typing_extensions import Literal
 
 import openai
 from openai import error, util, version

setup.py

@@ -25,7 +25,7 @@
         "pandas-stubs>=1.1.0.11",  # Needed for type hints for mypy
         "openpyxl>=3.0.7",  # Needed for CLI fine-tuning data preparation tool xlsx format
         "numpy",
-        "typing_extensions",  # Needed for type hints for mypy
+        'typing_extensions;python_version<"3.8"',  # Needed for type hints for mypy
     ],
     extras_require={
         "dev": ["black~=21.6b0", "pytest==6.*"],