add support for Python3.8

[gdementen]

It seems like all our dependencies now have conda packages for Python 3.8, so let's add official support for it.

[gdementen]

I moved this to the 0.33 milestone. Not really a fix and, even though it works fine on my machine, I am a bit nervous to announce official support for it without testing it on travis-ci first, but I cannot easily do that now as only the master branch is tested (and it even actively refuses any manual request to test another branch until the config is changed... but the config is in the master branch... doh).