configure.ac: post-release version bump to 3.9.1
configure.ac: post-release version bump to 3.9.1

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