Make typing_extensions conditional to Python < 3.8

All the typing features used by redis-py are available in Python 3.8+,
so typing_extensions is not used at all in newer versions of Python.
Adjust the dependencies accordingly.

Author: mgorny

requirements.txt

@@ -1,4 +1,4 @@
 async-timeout>=4.0.2
 deprecated>=1.2.3
 packaging>=20.4
-typing-extensions
+typing-extensions; python_version<"3.8"

setup.py

@@ -35,7 +35,7 @@
         "deprecated>=1.2.3",
         "packaging>=20.4",
         'importlib-metadata >= 1.0; python_version < "3.8"',
-        "typing-extensions",
+        'typing-extensions; python_version<"3.8"',
         "async-timeout>=4.0.2",
     ],
     classifiers=[