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 46 seconds (queued for 27 minutes and 28 seconds)
latest
Status Job ID Name Coverage
  Test
passed #207232224
test:3.5

00:01:17

100.0%
passed #207232225
test:3.6

00:01:12

100.0%
passed #207232226
test:3.7

00:01:17

100.0%