Fix REQUIRED_PYTHON in setup.py

Just a left-over from encode#8288 to sync the "Unsupported Python version" message with `python_requires`.

Author: sk1p

setup.py

@@ -8,7 +8,7 @@
 from setuptools import find_packages, setup
 
 CURRENT_PYTHON = sys.version_info[:2]
-REQUIRED_PYTHON = (3, 5)
+REQUIRED_PYTHON = (3, 6)
 
 # This check and everything above must remain compatible with Python 2.7.
 if CURRENT_PYTHON < REQUIRED_PYTHON: