Update CI pipeline

- Return to using the Python images, since Docker image cycle seems
  slow
- Add coverage report to py.test runs
- Add badge for coverage and update badge for pipeline
3 jobs for master in 5 minutes and 41 seconds (queued for 16 minutes and 13 seconds)
latest
Status Job ID Name Coverage
  Test
passed #215037621
test:3.5

00:02:44

100.0%
passed #215037622
test:3.6

00:01:41

100.0%
passed #215037623
test:3.7

00:01:15

100.0%