Python version is now restricted to 3.5 or 3.6. (#1145)

* Only Python 3.6, bumping package version.

ml-agents/setup.py

 
 setup(
     name='mlagents',
-    version='0.4.0',
+    version='0.5.0',
     description='Unity Machine Learning Agents',
     long_description=long_description,
     long_description_content_type='text/markdown',
         'pyyaml',
         'protobuf>=3.6,<3.7',
         'grpcio>=1.11.0,<1.12.0'],
+
+    python_requires=">=3.6,<3.7",
 
     entry_points={
         'console_scripts': [