Add a bespoke URL.with_fragment.

Clearly this doesn't fully solve #6, but we're going to incrementally
maintain immutability while adding out methods here.

src/lib.rs

@@ -175,6 +175,12 @@ impl UrlPy {
     fn cannot_be_a_base(&self) -> bool {
         self.inner.cannot_be_a_base()
     }
+
+    fn with_fragment(&self, fragment: Option<&str>) -> Self {
+        let mut cloned = self.inner.clone();
+        cloned.set_fragment(fragment);
+        UrlPy { inner: cloned }
+    }
 }
 
 #[repr(transparent)]

tests/test_url.py

@@ -80,6 +80,14 @@ def test_make_relative():
     assert relative == "c.png"
 
 
+def test_with_fragment():
+    url = URL.parse("https://example.com/data.csv")
+    assert url.fragment is None
+
+    evolved = url.with_fragment("foo")
+    assert (evolved.fragment, url.fragment) == ("foo", None)
+
+
 @pytest.mark.parametrize(
     "base_url",
     ["http://foo.com/bar", "http://foo.com/bar/"],