Checklist

I have read the contribution guidelines.
Unit tests have been added for any new or changed code, and the test suite passes. Note: Your request will likely not be merged without the appropriate tests.
"closes #XXXX" is in the body of the MR description to link the related issue, if applicable.

Master in this morning:

With this MR:

Can we probably turn on Appvoyer again also in the main ASE repository? (Or would we actually like to omit it?)

Bump to python3.8 in appveyor.yml