gh-117394: Speed up os.path.ismount() on Posix (GH-117447)

It is now 2-3 times faster if the user has permissions.

Lib/posixpath.py

@@ -206,11 +206,14 @@ def ismount(path):
         parent = join(path, b'..')
     else:
         parent = join(path, '..')
-    parent = realpath(parent)
     try:
         s2 = os.lstat(parent)
-    except (OSError, ValueError):
-        return False
+    except OSError:
+        parent = realpath(parent)
+        try:
+            s2 = os.lstat(parent)
+        except OSError:
+            return False
 
     # path/.. on a different device as path or the same i-node as path
     return s1.st_dev != s2.st_dev or s1.st_ino == s2.st_ino

Misc/NEWS.d/next/Library/2024-04-02-11-17-44.gh-issue-117394.2aoSlb.rst

@@ -0,0 +1 @@
+:func:`os.path.ismount` is now 2-3 times faster if the user has permissions.