test: Pass strings to readlink for Windows Python 3.7

The dangling-metadata regression passed pathlib.Path objects to
os.readlink() when capturing and checking the symlink target. Windows
Python 3.7 requires a string argument, so all four parameter combinations
failed with TypeError before exercising the clone fix.

Convert the path to str at both calls. The production clone helper already
passes a string and needs no change. Keep the target-preservation
assertions and the absolute/relative and realpath-mode coverage intact.

Validation: reproduced all four TypeErrors on Windows/Python 3.10 with an
in-memory readlink wrapper enforcing the Python 3.7 string requirement.
After the conversions, all four cases passed with the same wrapper and
Windows permission-error suppression disabled. Ruff 0.16.5 lint and
formatting checks and git diff --check passed. Native Python 3.7 was not
available locally.

Author: Byron

test/test_submodule.py

@@ -457,12 +457,12 @@ def test_add_to_dangling_metadata_symlink(movable_submodule, tmp_path, metadata_
     link = Path(sm.repo.git_dir) / "modules/new"
     target = tmp_path / "missing" / "metadata"
     link.symlink_to(osp.relpath(target, link.parent) if relative_target else target, target_is_directory=True)
-    link_target = os.readlink(link)
+    link_target = os.readlink(str(link))
 
     added = Submodule.add(sm.repo, "new", "new", sm.url)
 
     assert link.is_symlink() and link.is_dir()
-    assert os.readlink(link) == link_target
+    assert os.readlink(str(link)) == link_target
     assert (target / "HEAD").is_file()
     with added.module() as module:
         assert Path(module.git_dir).resolve() == target.resolve()