upgrade python for doing the math fork to python3.13

[miguelhisojo]

added python3.13 support

[rschwarz]

The changes look good to me, but I'd prefer to test it (manually) with Python 3.13 on my local machine.

[rschwarz]

I also wonder: on the PR to the upstream python-mip, ie, coin-or#415, there were additional changes in the following commit, that also seem to be related to Python 3.13, for example, in mip/lists.py:

if key is None:
    raise KeyError("Empty key access (e.g., varlist[]) is not allowed in Python 3.13")

Did this turn out not to be necessary?