[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/trainable.py

@@ -74,8 +74,7 @@ class TrainableUtil:
     @staticmethod
     def make_checkpoint_dir(checkpoint_dir):
         """Creates a checkpoint directory at the provided path."""
-        if not os.path.exists(checkpoint_dir):
-            os.makedirs(checkpoint_dir)
+        os.makedirs(checkpoint_dir, exist_ok=True)
         # Drop marker in directory to identify it as a checkpoint dir.
         open(os.path.join(checkpoint_dir, ".is_checkpoint"), "a").close()
 
@@ -127,8 +126,7 @@ class Trainable:
             self._logdir = self._result_logger.logdir
         else:
             logdir_prefix = datetime.today().strftime("%Y-%m-%d_%H-%M-%S")
-            ray.utils.try_to_create_directory(
-                DEFAULT_RESULTS_DIR, warn_if_exist=False)
+            ray.utils.try_to_create_directory(DEFAULT_RESULTS_DIR)
             self._logdir = tempfile.mkdtemp(
                 prefix=logdir_prefix, dir=DEFAULT_RESULTS_DIR)
             self._result_logger = UnifiedLogger(
@@ -411,8 +409,7 @@ class Trainable:
             path = os.path.join(tmpdir, relpath_name)
 
             # This may be a subdirectory, hence not just using tmpdir
-            if not os.path.exists(os.path.dirname(path)):
-                os.makedirs(os.path.dirname(path))
+            os.makedirs(os.path.dirname(path), exist_ok=True)
             with open(path, "wb") as f:
                 f.write(file_contents)