[tune] Remove py2.7-specific code (#6665)

* Remove backwards compatability py2.7 code.

* Use exists_ok=True in ray

* nit

* nit

Co-authored-by: Richard Liaw <rliaw@berkeley.edu>

python/ray/tune/syncer.py

@@ -7,10 +7,7 @@ import logging
 import os
 import time
 
-try:  # py3
-    from shlex import quote
-except ImportError:  # py2
-    from pipes import quote
+from shlex import quote
 
 from ray import services
 from ray.tune.cluster_info import get_ssh_key, get_ssh_user