Snapshot: don't walk directories without read permissions

Original patch by Joshua Skelton (@joshuaskelly) on issue gorakhargosh#408.

src/watchdog/utils/dirsnapshot.py

@@ -220,7 +220,7 @@ def walk(root):
                 # list of its parent and trying to delete its contents. If this
                 # happens we treat it as empty. Likewise if the directory was replaced
                 # with a file of the same name (less likely, but possible).
-                if e.errno == errno.ENOENT or e.errno == errno.ENOTDIR:
+                if e.errno in (errno.ENOENT, errno.ENOTDIR):
                     return
                 else:
                     raise
@@ -234,9 +234,16 @@ def walk(root):
                 yield _
             if recursive:
                 for path, st in entries:
-                    if S_ISDIR(st.st_mode):
-                        for _ in walk(path):
-                            yield _
+                    try:
+                        if S_ISDIR(st.st_mode):
+                            for _ in walk(path):
+                                yield _
+                    except (IOError, OSError) as e:
+                        # IOError for Python 2
+                        # OSError for Python 3
+                        # (should be only PermissionError when dropping Python 2 support)
+                        if e.errno != errno.EACCES:
+                            raise
 
         for p, st in walk(path):
             i = (st.st_ino, st.st_dev)