[tests] Add Python 3.8 to the Appveyor test matrix
[tests] Add Python 3.8 to the Appveyor test matrix

Python 3.8 is preinstalled at Appveyor:
https://www.appveyor.com/docs/windows-images-software/#python

Change-Id: I2921d4550abb4c32815bae9ba84a01e37e675966

PersianWikipedia/pywikibot-core,wikimedia/pywikibot-core,wikimedia/pywikibot-core