user/gtk-doc: always use 'python3' as PYTHON

user/gtk-doc/APKBUILD

 # Maintainer: 
 pkgname=gtk-doc
 pkgver=1.29
-pkgrel=0
+pkgrel=1
 pkgdesc="Documentation tool for public library API"
 url="https://www.gtk.org/gtk-doc/"
 arch="noarch"
@@ -23,6 +23,7 @@ prepare() {
 
 build() {
 	cd "$builddir"
+	export PYTHON=python3
 	./configure \
 		--build=$CBUILD \
 		--host=$CHOST \