Point release

0.21.3

Deprecations

• Constructing {class}fwdpy11.ModelParams without a demographic object is now deprecated
  and raises a warning.
  This will be a hard error in a future release.
  PR {pr}1219

Documentation

• Minor updates to manual.
  Some formatting issues fixed.
  PR {pr}1222
  PR {pr}1223

Python back end

• Use tskit.NODE_IS_SAMPLE constant instead of relying on the numeric value
  when exporting to tskit.
  PR {pr}1218