fix: replace np.int alias with builtin int

[marvinvanaalst]

This changes the np.int alias of the Python builtin int type as the alias was deprecated in numpy version 1.20.0 and lead to an AttributeError starting with version 1.24.0.

This should fix issue #104

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[vineetbansal]

@marvinvanaalst, and sorry for the delay in getting to this, and thanks a bunch for the PR.

2 things before we merge this in:

• Can you undo the change in pyproject.toml regarding the black configuration? (since its outside the scope of this PR).
• It looks to me like the alias np.int pointing to int has been in numpy since version 1.7, the current min. version of numpy for osqp. So while I agree that np.int should be replaced with int, there's no need to bump up the required numpy version. If you can change that back in requirements.txt that would be the right thing to do.

[vineetbansal]

@marvinvanaalst - after approving PR #109 with this fix, I'm closing this PR. Release v0.6.2.post9 of osqp incorporates these changes. Thanks for bringing this to our attention!