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 2 minutes and 41 seconds (queued for 5 minutes and 38 seconds)
latest
Status Job ID Name Coverage
  Test
passed #210963392
test:3.5

00:01:18

100.0%
passed #210963393
test:3.6

00:01:22

100.0%
passed #210963394
test:3.7

00:01:20

100.0%