Improve github actions

[ckleemann]

I did some improvements on the action definition. In the end we have speedup of ~40% e.g. from 1:29 minutes to 51 seconds
Changes:

• Actions now run for push and pull_requests on every branch
• Build was renamed to test
• The Ubuntu version is pinned to a specific version instead of "latest". (In January 2021 "latest" is still 18.04)
• Added python 3.9
• Added caching for pip
• For test run only md5 is used. Only this saves 12 seconds
• Split tests in Static tests and Unit test
• Static tests also check for missing migrations
• remove duplicate django test run. coverage run also outputs errors
• move coverage settings to .coveragerc file

I think it is now a good blueprint which also could by used for pyophase.

[miterion]

lgtm