rmtree: fix rmtree with broken symlinks on python >= 3.8

After https://github.com/python/cpython/issues/86180 func is os.open

src/zc/buildout/rmtree.py

@@ -102,7 +102,7 @@ def rmtree (path):
         """
         if func is os.path.islink:
             os.unlink(path)
-        elif func is os.lstat:
+        elif func is os.lstat or func is os.open:
             if not os.path.islink(path):
                 raise
             os.unlink(path)