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 3 minutes and 59 seconds (queued for 10 minutes and 42 seconds)
latest
Status Job ID Name Coverage
  Test
passed #203759684
test:3.5

00:01:24

100.0%
passed #203759685
test:3.6

00:01:20

100.0%
passed #203759686
test:3.7

00:01:15

100.0%