Use pickle by default for serialization (#5978)

python/ray/__init__.py

@@ -5,12 +5,26 @@ from __future__ import print_function
 import os
 import sys
 
+# MUST add pickle5 to the import path because it will be imported by some
+# raylet modules.
+
+if "pickle5" in sys.modules:
+    raise ImportError("Ray must be imported before pickle5 because Ray "
+                      "requires a specific version of pickle5 (which is "
+                      "packaged along with Ray).")
+
+# Add the directory containing pickle5 to the Python path so that we find the
+# pickle5 version packaged with ray and not a pre-existing pickle5.
+pickle5_path = os.path.join(
+    os.path.abspath(os.path.dirname(__file__)), "pickle5_files")
+sys.path.insert(0, pickle5_path)
+
 # MUST import ray._raylet before pyarrow to initialize some global variables.
 # It seems the library related to memory allocation in pyarrow will destroy the
 # initialization of grpc if we import pyarrow at first.
 # NOTE(JoeyJiang): See https://github.com/ray-project/ray/issues/5219 for more
 # details.
-import ray._raylet
+import ray._raylet  # noqa: E402
 
 if "pyarrow" in sys.modules:
     raise ImportError("Ray must be imported before pyarrow because Ray "