Update python_requires in setup.py

I am really happy to make this nice and short.

setup.py

@@ -85,5 +85,5 @@ setup(
     },
 
     zip_safe=False,
-    python_requires='>=2.7,!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.*,!=3.4.*',
+    python_requires='>=3.6',
 )