Refresh the CI
This MR refreshes the CI as follows
- use
extends
instead of<<:
- import templates from https://computing.docs.ligo.org/gitlab-ci-templates/
- split out build/test jobs for all relevant OS versions
- stretch
- buster
- bullseye
- el7
- el8
- run the test scripts using pytest as a harness - it has better reporting
Edited by Duncan Macleod