Add python_requires setuptools check for python > 3.6 (#1656)

Author: Nicusor97

setup.py

@@ -21,6 +21,7 @@
     url="https://github.com/redis/redis-py",
     author="Redis Inc.",
     author_email="[email protected]",
+    python_requires=">=3.6",
     classifiers=[
         "Development Status :: 5 - Production/Stable",
         "Environment :: Console",