Switch to using GitLab CI extends

[omus]

Use GitLab CI's extends instead of YAML anchors. The extends is a bit more flexible and the syntax is a bit cleaner.

Note this change will conflict with #131.

[codecov]

Codecov Report

Merging #140 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #140   +/-   ##
=======================================
  Coverage   93.61%   93.61%           
=======================================
  Files          17       17           
  Lines         329      329           
=======================================
  Hits          308      308           
  Misses         21       21           

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[christopher-dG]

Needs a rebase but looks good to me, provided you've tested that this actually works properly with GitLab!

[omus]

Rebased and I've validated that the change works with GitLab CI. I'll note that extends requires GitLab 11.3 which we may want to mention somewhere. Current version is 12.8