Use scandir to save memory

src/watchdog/utils/dirsnapshot.py

@@ -48,6 +48,10 @@
 import os
 from stat import S_ISDIR
 from watchdog.utils import stat as default_stat
+try:
+    from os import scandir as listdir
+except ImportError:
+    from os import listdir
 
 
 class DirectorySnapshotDiff(object):
@@ -199,7 +203,7 @@ class DirectorySnapshot(object):
     def __init__(self, path, recursive=True,
                  walker_callback=(lambda p, s: None),
                  stat=default_stat,
-                 listdir=os.listdir):
+                 listdir=listdir):
         self._stat_info = {}
         self._inode_to_path = {}
 
@@ -209,7 +213,8 @@ def __init__(self, path, recursive=True,
 
         def walk(root):
             try:
-                paths = [os.path.join(root, name) for name in listdir(root)]
+                paths = [os.path.join(root, entry if isinstance(entry, str) else entry.name)
+                         for entry in listdir(root)]
             except OSError as e:
                 # Directory may have been deleted between finding it in the directory
                 # list of its parent and trying to delete its contents. If this