ci: Pin wxWidgets on AppVeyor

At the moment, wxPython is broken on 64-bit Windows with the latest
wxWidgets build in conda-forge:
conda-forge/wxpython-feedstock#125

Author: QuLogic

.appveyor.yml

@@ -57,7 +57,8 @@ init:
   - micromamba config prepend channels conda-forge
 
 install:
-  - micromamba env create -f environment.yml python=%PYTHON_VERSION% pywin32
+  # Pin wxwidgets due to https://github.com/conda-forge/wxpython-feedstock/issues/125
+  - micromamba env create -f environment.yml python=%PYTHON_VERSION% pywin32 wxwidgets=3.2.5
   - micromamba activate mpl-dev
 
 test_script: