Various improvements to the HiGHS interface

[rschwarz]

High-level summary:

• switched binary library from highspy to highsbox, because highspy was not meant to be used that way, which caused problems on some platforms
• bugfixes in var_get_index/constr_get_index, when a user asks for a variable/constraint with an invalid name
• bugfix in set_objective, where we now first reset the objective with all-zero coefficients
• bugfix in get_var_type, with work-around for purely continuous problems
• added more tests
• allow for Python 3.13 (not specific to HiGHS)

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[rschwarz]

When I ran tests locally, I got errors (actually crashes) with CBC, but if I just test

pytest -k "not CBC" test/test_model.py test/mip_test.py

It all works fine.

EDIT: See

$ pytest -k CBC test/test_model.py 
===== test session starts =====
platform linux -- Python 3.12.3, pytest-8.4.2, pluggy-1.6.0
rootdir: /home/robert/src/python-mip
configfile: pyproject.toml
plugins: xdist-3.8.0
collected 278 items / 139 deselected / 139 selected                                                                                                                                                                     

test/test_model.py .Aborted (core dumped)

[rschwarz]

I've also made a change to the pre-commit configuration (upgrade flake8) to avoid an error that happens during the first of the checks.

[h-g-s]

Hi @rschwarz . Thanks for the contributions and sorry for the delay. Merging to master.