[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/checkpoint_manager.py

@@ -6,11 +6,6 @@ from __future__ import print_function
 import heapq
 import logging
 
-try:
-    FileNotFoundError
-except NameError:
-    FileNotFoundError = IOError
-
 logger = logging.getLogger(__name__)
 
 
@@ -44,12 +39,6 @@ class QueueItem:
         self.priority = priority
         self.value = value
 
-    def __cmp__(self, other):
-        # For python2.7 compatibility.
-        if self.priority == other.priority:
-            return 0
-        return -1 if self.priority < other.priority else 1
-
     def __lt__(self, other):
         return self.priority < other.priority