travis: remove python 2.7 and add python 3.8
travis: remove python 2.7 and add python 3.8

Brown-University-Library/django-shibboleth-remoteuser,Brown-University-Library/django-shibboleth-remoteuser,trevoriancox/django-shibboleth-remoteuser,trevoriancox/django-shibboleth-remoteuser