flav-io/flavio v0.21.2

This minor release brings two small improvements and corresponds to the numerics used in the published version of arXiv:1703.09189. Improvements Correlations between LHCb measurements of B_s→φll angular observables added (thanks to Javier Virto for suggesting and to @peterstangl for implementing it). In the last release, the integration precision for theoretically clean lepton flavour universality tests was increased; this was now implemented also for B_s→φll