gh-63283: IDNA prefix should be case insensitive (GH-17726)

Any capitalization of "xn--" should be acceptable for the ACE prefix
(see https://tools.ietf.org/html/rfc3490#section-5).

Co-authored-by: Pepijn de Vos <pepijndevos@gmail.com>
Co-authored-by: Erlend E. Aasland <erlend@python.org>
Co-authored-by: Petr Viktorin <encukou@gmail.com>