test: add pytest missing plugins

[12rambau]

Final touch to the pytest plugins: Fix #630

pytest-deadfixture

• install pytest-deadfixture in setup.py
• use it the unit test suit against python 3.8 (example of failure: https://github.com/12rambau/sepal_ui/actions/runs/3571282698/jobs/6003017612)
• tune test_PlanetModel to expose the use of cred and planet_api

pytest-cov

• install pytest-cov in setup.py
• use it in our unit test suit to generate the xml coverage file

[codecov]

Codecov Report

Merging #639 (84a5f0b) into main (8a8196e) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #639   +/-   ##
=======================================
  Coverage   92.63%   92.63%           
=======================================
  Files          36       36           
  Lines        3911     3911           
=======================================
  Hits         3623     3623           
  Misses        288      288           

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[12rambau]

I need this PR to be merged for the conda release. I'm overpassing review checks. Don't hesitate to reach in the issues if needed