Release version 1.6

* Breaking change in spline EOS file format
  EOS of type eos_barotr_spline are now interpolating the soundspeed as
  function of density instead pseudo-enthalpy, because otherwise
  phase transitions cannot be represented. This required a change in
  the file format for this EOS type as well. Old files of type
  eos_barotr_spline cannot be read anymore. Files of all other EOS types
  are unaffected.
* The Python API can now be build also for Python 3.11

Author: Wolfgang Kastaun