Remove unused codecov dependency

[rly]

Motivation

The codecov package on PyPI was deprecated and suddenly deleted today. See https://community.codecov.com/t/codecov-yanked-from-pypi-all-versions/4259 and https://about.codecov.io/blog/message-regarding-the-pypi-package/

We no longer use the CLI for codecov so we can safely remove it as a dependency in requirements-dev.txt

Checklist

• Did you update CHANGELOG.md with your changes?
• Have you checked our Contributing document?
• Have you ensured the PR clearly describes the problem and the solution?
• Is your contribution compliant with our coding style? This can be checked running flake8 from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.

[codecov]

Codecov Report

Merging #1688 (4947b99) into dev (de7f474) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##              dev    #1688    +/-   ##
========================================
  Coverage   91.93%   91.93%            
========================================
  Files          26       26            
  Lines        2592     2592            
  Branches      477      679   +202     
========================================
  Hits         2383     2383            
  Misses        134      134            
  Partials       75       75            

Flag	Coverage Δ
integration	71.52% <ø> (ø)
unit	83.56% <ø> (ø)

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