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 52 seconds (queued for 7 minutes and 38 seconds)
latest
Status Job ID Name Coverage
  Test
passed #192055829
test:3.5

00:01:15

100.0%
passed #192055830
test:3.6

00:01:16

100.0%
passed #192055831
test:3.7

00:01:20

100.0%