Fix stestr and mypy issues

[t-imamichi]

Summary

Fix "stestr run" error by pinning python-subunit version.
Update stestr version that has been just released.
https://github.com/mtreinish/stestr/releases/tag/4.1.0

Fix mypy by modifying colormap access.

I found these errors in the recent scheduled CI.
https://github.com/qiskit-community/qiskit-optimization/actions/runs/6217275395

Details and comments

[woodsp-ibm]

I guess this is fine - the constraint change that Qiskit did was simply closed. CI pulls in the latest version anyway even with >=2.0.0, but if you had some local older stestr install working this requires things to be updated if you pip install this dev file. Qiskit left their requirements-dev as it was before. I guess if you had things working locally and somehow just updated python-subunit then it would faili if you did not have anything installed then as of now, like CI, it would install 4.1.0 and work.

[t-imamichi]

Yes, I updated the version because someone may update only python-subunit and have the stestr trouble.

[woodsp-ibm]

I put a comment in there - I think its ok like this but I'll leave it to you.

[woodsp-ibm]

@t-imamichi I was looking through the combined deprecation messages from the build, as this was the latest. I noticed this which seems related to what you fixed here but elsewhere

/home/runner/work/qiskit-optimization/qiskit-optimization/qiskit_optimization/applications/bin_packing.py:121: MatplotlibDeprecationWarning: The get_cmap function was deprecated in Matplotlib 3.7 and will be removed two minor releases later. Use matplotlib.colormaps[name] or matplotlib.colormaps.get_cmap(obj) instead.

[t-imamichi]

Thanks for the report. I found a line that uses get_cmap. I will fix it.

qiskit-optimization/qiskit_optimization/applications/bin_packing.py

Line 121 in e4804af

colors = plt.cm.get_cmap("jet", len(self._weights))