Use PurePosixPath for relative paths within a zip file. Fixes failing…

… test on Windows. Added test for relative paths with subdirectories.

tests/test_zipp.py

@@ -444,7 +444,10 @@ def test_is_symlink(self, alpharep):
     def test_relative_to(self, alpharep):
         root = zipp.Path(alpharep)
         relative = root.joinpath("b", "c.txt").relative_to(root / "b")
-        assert relative == pathlib.Path("c.txt")
+        assert relative == pathlib.PurePosixPath("c.txt")
+
+        relative = root.joinpath("b", "d", "e.txt").relative_to(root / "b")
+        assert str(relative) == "d/e.txt"
 
     @pass_alpharep
     def test_inheritance(self, alpharep):

zipp/__init__.py

@@ -354,8 +354,8 @@ def rglob(self, pattern):
         return self.glob(f'**/{pattern}')
 
     def relative_to(self, *other, **kwargs):
-        other = (pathlib.Path(each.at) for each in other)
-        return pathlib.Path(self.at).relative_to(*other, **kwargs)
+        other = (pathlib.PurePosixPath(each.at) for each in other)
+        return pathlib.PurePosixPath(self.at).relative_to(*other, **kwargs)
 
     def __str__(self):
         return posixpath.join(self.root.filename, self.at)