pygtk: modernize optional dependency

Closes #28092.

Note from Jack:

I know renaming an option is bad form, but my gut says this is
a little-used option and it's worth changing to fix the cases where it
is causing breakage.

See https://github.com/Homebrew/homebrew/issues/27485 for background.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>