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 42 seconds (queued for 6 minutes and 13 seconds)
latest
Status Job ID Name Coverage
  Test
passed #183054089
test:3.5

00:01:19

100.0%
passed #183054090
test:3.6

00:01:19

100.0%
passed #183054091
test:3.7

00:01:23

100.0%