makedirs: add comment on why do have to use umask

dvc/utils/__init__.py

@@ -152,6 +152,9 @@ def makedirs(path, exist_ok=False, mode=None):
         _makedirs(path, exist_ok=exist_ok)
         return
 
+    # utilize umask to set proper permissions since Python 3.7 the `mode`
+    # `makedirs` argument no longer affects the file permission bits of
+    # newly-created intermediate-level directories.
     umask = os.umask(0o777 - mode)
     try:
         _makedirs(path, exist_ok=exist_ok)