flav-io/flavio v0.21.1

This minor release fixes two problems:

• It increases the numerical integration precision for observables that have a very small SM uncertainty (like the lepton flavour universality ratios in B decays), where this precision is relevant
• It corrects the renormalization scale used for the computation of the SM contribution to meson-antimeson mixing, which previously was not consistent with the QCD correction factors used (thanks to Jacky Kumar for pointing out this problem)