cython release 3.1.6+dfsg-2 for unstable

[dgit distro=debian split --quilt=gbp]
[dgit please-upload source=cython version=3.1.6+dfsg-2 upstream-tag=upstream/3.1.6+dfsg upstream=7e3ce9ccf63e7cb0693c0684e4919ea4240358a8]