Add hack to build wx on windows

.appveyor.yml

@@ -40,6 +40,8 @@ install:
   # pip will build them from source using the MSVC compiler matching the
   # target Python version and architecture
   - ps: echo("Install pip requirements:")
+  # This one is needed to build wxpython 4.0.6 on windows
+  - cmd: "python -m pip install pathlib"
   - cmd: "python -m pip install -r requirements.txt"
   - cmd: "python -m pip install PyInstaller==3.6"