Update python_requires to require 3.6+ (#83)

This matches what's defined in the classifiers.

setup.py

@@ -9,7 +9,7 @@
 
 
 package = 'ipware'
-python_requires = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*, !=3.5.*"
+python_requires = ">=3.6"
 here = os.path.abspath(os.path.dirname(__file__))
 
 requires = []