Provide Windows compilation with Python 3.8, switch Appveyor to it
|Status:||Resolved||Start date:||11 May 2020|
|Target version:||Sprint 43|
While switching to Python 3.8, Appveyor builds (and also local Windows builds) fail during the execution of the PyStandard test family. The reason is failed import bornagain because of "failed import of _libBornAgainCore.pyd". The reason is unclear and will be investigated later.