fix: activate venv instead of using poetry run

[sehyod]

I think we had some issues with our cache and removing it fixed the failing CI.

To delete cache, you can go there. The one that had an issue here was venv-Linux-..., the cache for poetry dependencies. We are also caching yarn (node-cache-Linux-yarn-...) and now poetry with this PR (poetry-cache-...)

This PR is removing the unnecessary installation of pytest via pip (pytest is installed via poetry in setup-python)
This is also improving our CI by caching poetry so that python jobs now take half of the time they use to take:

Before the PR:

After the PR:

[codecov]

Codecov Report

Merging #518 (fc9639b) into main (df40d2b) will not change coverage.
Report is 1 commits behind head on main.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #518   +/-   ##
=======================================
  Coverage   81.65%   81.65%           
=======================================
  Files         213      213           
  Lines       12098    12098           
  Branches     1179     1179           
=======================================
  Hits         9879     9879           
  Misses       2205     2205           
  Partials       14       14           

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