10631: Get CircleCI to store Python coverage report

[stevejalim]

Description

Now we have coverage.py running again, we want to surface the metrics at CI level.

This changeset stores the output of the coverage checks as a CircleCI artefact.

We then use CodeCov to report on the result. The CodeCov integration is powered by their own Orb, which runs verification checks on the uploader script to ensure its integrity - you can see those steps happen in CircleCI

Issue / Bugzilla link

#10631

Testing

• This branch should go through CircleCI for tests and result in a python_coverage dir existing in the artefacts for the build.
• This resulting coverage data should be piped via CodeCov and trigger a custom message, below, reporting current coverage status
• There should be no failure conditions tied to coverage for now - let's leave humans decide

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (master@35e60ae). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master   #10640   +/-   ##
=========================================
  Coverage          ?   69.42%           
=========================================
  Files             ?      149           
  Lines             ?     8402           
  Branches          ?        0           
=========================================
  Hits              ?     5833           
  Misses            ?     2569           
  Partials          ?        0           

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[stevejalim]

@robhudson @pmac So we've got coverage metrics being pumped back into the PR. This one lacks a base commit to compare to, so we might find we need to do a bit of tuning, but there is (deliberately) no config that fails CI or similar based on a change in coverage so I (famous last words) think we won't get any blocked PRs because of this. We can look to add strict checks in the future, if we decide it's wortwhile.

[stevejalim]

Am 95% sure that the codecov.yml provided will show us a coverage chart every time CI completes, but I see that the one in the PR above hasn't updated, so I will keep an eye on this aspect once this is merged in case it needs tuning

[pmac]

Looks great. Let's give it a try 🍰

[robhudson]

Excited to start seeing these!