Apply titlecase mapping in str.title() for uppercase digraphs (#7748)

The uppercase/titlecase branch of PyStr::title() pushed characters
unchanged when starting a new word, which left Latin Extended-B
digraphs (U+01F1 'DZ', U+01C4 'DŽ', etc.) in their uppercase form
instead of mapping them to their distinct titlecase counterparts
(U+01F2 'Dz', U+01C5 'Dž'). For ASCII letters and characters where
to_titlecase is identity this had no effect, hiding the bug for the
common case.

Mirror the lowercase branch — which already calls to_titlecase()
when starting a new word — so both branches symmetrically apply
the titlecase mapping. char::to_titlecase is identity for already-
titlecase and ASCII-uppercase characters, so existing cases stay
correct.

Also unmasks test_unicodedata.UnicodeMiscTest.test_bug_4971, which
asserts exactly this behavior (`'DŽ'.title() == 'Dž'` etc.)
and was marked expectedFailure with reason `+ Dž`.

Closes #7527 (the only example from that issue still failing on
3.14.4; the other four examples already pass on current main).

Lib/test/test_unicodedata.py

@@ -359,7 +359,6 @@ class UnicodeMiscTest(UnicodeDatabaseTest):
             [0]
         )
 
-    @unittest.expectedFailure  # TODO: RUSTPYTHON; + Dž
     def test_bug_4971(self):
         # LETTER DZ WITH CARON: DZ, Dz, dz
         self.assertEqual("\u01c4".title(), "\u01c5")

crates/vm/src/builtins/str.rs

@@ -1053,7 +1053,7 @@ impl PyStr {
                 if previous_is_cased {
                     title.extend(c.to_lowercase());
                 } else {
-                    title.push_char(c);
+                    title.extend(c.to_titlecase());
                 }
                 previous_is_cased = true;
             } else {
@@ -2661,6 +2661,10 @@ mod tests {
             ("Greek Ωppercases ...", "greek ωppercases ..."),
             // spell-checker:disable-next-line
             ("Greek ῼitlecases ...", "greek ῳitlecases ..."),
+            // Latin Extended-B digraphs: uppercase forms map to titlecase forms
+            // (e.g. U+01F1 'DZ' -> U+01F2 'Dz', U+01C4 'DŽ' -> U+01C5 'Dž').
+            ("\u{01F2}", "\u{01F1}"),
+            ("\u{01C5}", "\u{01C4}"),
         ];
         for (title, input) in tests {
             assert_eq!(PyStr::from(input).title().as_str(), Ok(title));