fix: Swapcase must handle multibyte expansions (#7559)

`swapcase` used `to_ascii_lowercase` and uppercase to swap cases. This
is fine for ASCII, but code points may expand into multiple bytes which
leads to incorrect case swaps for some languages. The fix is to use
`to_lowercase` and `to_uppercase` instead.

Unfortunately, this leads to a realloc in `swapcase` when bytes are
expanded.

Part of #7526.

crates/vm/src/builtins/str.rs

@@ -1045,11 +1045,11 @@ impl PyStr {
         let mut swapped_str = Wtf8Buf::with_capacity(self.data.len());
         for c_orig in self.as_wtf8().code_points() {
             let c = c_orig.to_char_lossy();
-            // to_uppercase returns an iterator, to_ascii_uppercase returns the char
+            // to_uppercase returns an iterator because case changes may be multiple bytes
             if c.is_lowercase() {
-                swapped_str.push_char(c.to_ascii_uppercase());
+                swapped_str.extend(c.to_uppercase());
             } else if c.is_uppercase() {
-                swapped_str.push_char(c.to_ascii_lowercase());
+                swapped_str.extend(c.to_lowercase());
             } else {
                 swapped_str.push(c_orig);
             }

extra_tests/snippets/builtin_str.py

@@ -235,6 +235,8 @@ assert "유니코드".isidentifier()
 assert not "😂".isidentifier()
 assert not "123".isidentifier()
 
+assert "Σίσυφος".swapcase() == "σΊΣΥΦΟΣ"
+
 # String Formatting
 assert "{} {}".format(1, 2) == "1 2"
 assert "{0} {1}".format(2, 3) == "2 3"