Explicitly declare dtypes in numpy arrays

[andodet]

Apparently from numpy==1.24 it is not possible to create an array with elements of different length without explicitly passing the dytpe=object parameter.

This PR does the following:

• Pass the dytpe parameter anywhere needed
• Same as above for sample arrays used in tests
• I've been forced to run black on the changed files to be compliant with pre-commit hooks so this PR might contain lines that are not really related to this PR.

This allows to start updating numpy again. I am not sure whether Pipfile.lock should be updated accordingly.

Closes #75

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (34cdf90) 98.40% compared to head (1e6c1b9) 98.40%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #90 +/- ## ======================================= Coverage 98.40% 98.40% ======================================= Files 8 8 Lines 566 566 ======================================= Hits 557 557 Misses 9 9 ``` | [Impacted Files](https://app.codecov.io/gh/scikit-learn-contrib/hiclass/pull/90?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scikit-learn-contrib) | Coverage Δ | | |---|---|---| | [hiclass/HierarchicalClassifier.py](https://app.codecov.io/gh/scikit-learn-contrib/hiclass/pull/90?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=scikit-learn-contrib#diff-aGljbGFzcy9IaWVyYXJjaGljYWxDbGFzc2lmaWVyLnB5) | `95.54% <100.00%> (ø)` | |

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[mirand863]

Hi @andodet

Thank you so much for this pull request! I reviewed and it looks good to merge.

Best regards, Fabio