message: Extend build tests to Python 3.8
  Extend Appveyor build tests to include Python 3.8, which is supported by the latest version of PyBind11.
diff: @@ -10,11 +10,11 @@ environment:
  - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
  GENERATOR: Visual Studio 15 2017
  ARCH: Win32
  - PYTHON: C:\Python36
  + PYTHON: C:\Python37
  - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
  GENERATOR: Visual Studio 16 2019
  ARCH: x64
  - PYTHON: C:\Python37-x64
  + PYTHON: C:\Python38-x64
  configuration:
  - Release
  