Change os.getuid() guard in tmpdir.py

This matches the guard used by typeshed and avoid mypy errors on Windows:

https://github.com/python/typeshed/blob/48a346920bbc24165bd9e9cf29a2b5140eae2abc/stdlib/os/__init__.pyi#L388

Author: nicoddemus

src/_pytest/tmpdir.py

@@ -1,6 +1,7 @@
 """Support for providing temporary directories to test functions."""
 import os
 import re
+import sys
 import tempfile
 from pathlib import Path
 from typing import Optional
@@ -130,9 +131,9 @@ def getbasetemp(self) -> Path:
             # Also, to keep things private, fixup any world-readable temp
             # rootdir's permissions. Historically 0o755 was used, so we can't
             # just error out on this, at least for a while.
-            if hasattr(os, "getuid"):
-                rootdir_stat = rootdir.stat()
+            if sys.platform != "win32":
                 uid = os.getuid()
+                rootdir_stat = rootdir.stat()
                 # getuid shouldn't fail, but cpython defines such a case.
                 # Let's hope for the best.
                 if uid != -1: