Build the speedup C-extension against the stable API. This is done by emulating PyUnicode_New()/PyUnicode_WriteChar() with encoding the UCS4 character to UTF-8 and using PyUnicode_FromStringAndSize().

{"__comment1": "Comment 1",
 "key1": "the value",
 "key2": 2,
 "key3": 5.7,
 "key4": true,
 "key5": false,
 "key6": 255,
 "__comment2": "Comment no 2",
 "key7": "Umlaute: ÄÖÜäöüß"
}