MAINT: Revision of GHA migration

[oesteban]

Revises the GHA migration to make sure that tests run:

• Added dataset download using datalad (as it was the case in Circle)
• Improved the tox config to ensure environment variables are considered
• Added execution of pytest on the test/ folder.
• Does not test on a matrix of pythons because the python version should not introduce differences (to consider when we have code where it could be a problem).

IMHO, the addition of tox deviates from other projects (and as far as I understand, we were not planning on using tox) and has introduced some complexity. In particular, I had to set one test to skip because of (I suspect) some minimal environment change tox introduces.

For now, I wanted to focus on the download of data and execution of all tests. Future PRs can address the removal of tox if we finally decide against it.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 53.35%. Comparing base (a7880a3) to head (7fb31c8). Report is 5 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #230 +/- ## =========================================== + Coverage 32.97% 53.35% +20.37% =========================================== Files 17 17 Lines 849 849 Branches 149 143 -6 =========================================== + Hits 280 453 +173 + Misses 558 369 -189 - Partials 11 27 +16 ```

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