Second alpha of 0.20.0

0.20.0a1

New features

• Add {class}fwdpy11.BinomialIntervalMap.
  PR {pr}1142

Back end

• Improve test suite run times.
  PR {pr}1136
• Refactor some C++ test fixtures.
  PR {pr}1139
• Python tests now treat warnings as errors.
  PR {pr}1135

Documentation

• Improve docs related to {class}fwdpy11.ForwardDemesGraph.
  PR {pr}1133
  PR {pr}1137