[rllib] [RFC] Deprecate Python 2 / RLlib (#4832)

python/ray/rllib/__init__.py

@@ -3,6 +3,7 @@ from __future__ import division
 from __future__ import print_function
 
 import logging
+import sys
 
 # Note: do not introduce unnecessary library dependencies here, e.g. gym.
 # This file is imported from the tune module in order to register RLlib agents.
@@ -30,6 +31,11 @@ def _setup_logger():
     logger.addHandler(handler)
     logger.propagate = False
 
+    if sys.version_info[0] < 3:
+        logger.warn(
+            "RLlib Python 2 support is deprecated, and will be removed "
+            "in a future release.")
+
 
 def _register_all():