configure.ac: post-release bump to 3.9.90
configure.ac: post-release bump to 3.9.90

sfeltman/pygobject,GNOME/pygobject,sfeltman/pygobject,pexip/pygobject,GNOME/pygobject,GNOME/pygobject,davibe/pygobject,pexip/pygobject,pexip/pygobject,davibe/pygobject,davibe/pygobject,davibe/pygobject,sfeltman/pygobject