Pillow 9.5 deprecated the getsize method for fonts in favor of its
new getbbox, and then getsize was removed in Pillow 10.. Adjust the
call we use for captcha generation so it will continue working, and
declare a requirement for the earliest forward-compatible release.
Change-Id: Ib35eea077d7f8c428493ef8e4cfe00e05e580a6f