Change test_root_module Windows skip to xfail

And rewrite the reason to give more useful information. (The new
reason also doesn't state the exception type, because that is now
specified, and checked by pytest, by being passed as "raises".)

test/test_submodule.py

@@ -474,14 +474,13 @@ def test_base_bare(self, rwrepo):
         reason="Cygwin GitPython can't find submodule SHA",
         raises=ValueError,
     )
-    @skipIf(
+    @pytest.mark.xfail(
         HIDE_WINDOWS_KNOWN_ERRORS,
-        """
-        E  PermissionError:
-        [WinError 32] The process cannot access the file because it is being used by another process:
-        'C:\\Users\\ek\\AppData\\Local\\Temp\\non_bare_test_root_modulep0eqt8_r\\git\\ext\\gitdb'
-        -> 'C:\\Users\\ek\\AppData\\Local\\Temp\\non_bare_test_root_modulep0eqt8_r\\path\\prefix\\git\\ext\\gitdb'
-        """,
+        reason=(
+            '"The process cannot access the file because it is being used by another process"'
+            + " on first call to rm.update"
+        ),
+        raises=PermissionError,
     )
     @with_rw_repo(k_subm_current, bare=False)
     def test_root_module(self, rwrepo):