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 40 seconds (queued for 23 minutes and 50 seconds)
latest
Status Job ID Name Coverage
  Test
passed #187429892
test:3.5

00:01:19

100.0%
passed #187429893
test:3.6

00:01:15

100.0%
passed #187429894
test:3.7

00:01:21

100.0%