CI: Add dependabot config

[ghisvail]

Add dependabot configuration watching updates to GitHub actions.

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.05 ⚠️

Comparison is base (a85bb14) 81.77% compared to head (7e9584f) 81.72%.

❗ Current head 7e9584f differs from pull request most recent head 755355f. Consider uploading reports for the commit 755355f to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #633      +/-   ##
==========================================
- Coverage   81.77%   81.72%   -0.05%     
==========================================
  Files          20       20              
  Lines        4394     4394              
  Branches     1264        0    -1264     
==========================================
- Hits         3593     3591       -2     
- Misses        797      803       +6     
+ Partials        4        0       -4     

Flag	Coverage Δ
unittests	81.72% <ø> (-0.05%)	⬇️

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see 4 files with indirect coverage changes

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[ghisvail]

I wonder how to fix codecov's spurious complaints 🤔

[effigies]

Do we have some tests that only run on master, so PRs will always look like regressions?

[ghisvail]

Do we have some tests that only run on master, so PRs will always look like regressions?

I am not sure how to interpret what codecov is reporting here. I'll check with another PR.

It just sound weird to get coverage regression report on a PR adding a GHA.