AppVeyor Python 3.8 temporarily broken

As other CIs have done, AppVeyor updated to Python 3.8 in the default Linux, MacOS and Windows images. Since Python library wheels are often not immediately available for recently-released Python versions, this can lead to CI test nuisance errors at setup. Many projects will experience AppVeyor errors on Numpy setup as AppVeyor attempts to compile Numpy since the wheel doesn’t exist yet for pre-release Python versions.

Select Python version

AppVeyor has several Python versions installed. The AppVeyor Python version selection method is OS-dependent.

The examples are shown specific to Windows and Linux for didactic clarity. They would normally be merged to a single .appveyor.yml.

Windows builds

Select Python version on AppVeyor Windows CI by setting .appveyor.yml to the desired AppVeyor Windows Python version directory:

image:
- Visual Studio 2019

environment:
  PY_DIR: C:/Python38-x64

init:
- cmd: set PATH=%PY_DIR%;%PY_DIR%\Scripts;%PATH%

Linux builds

Select Python version on AppVeyor Linux CI by setting .appveyor.yml to the desired AppVeyor Linux Python version directory:

image:
- Ubuntu

stack: python 3