Fix CI warnings

[murrayrm]

This PR makes a bunch of small changes to remove warning messages in the GitHub actions CI runs:

• Access scalar NumPy arrays using item() (old usage is newly deprecated in NumPy).
• Change set_tight_layout(True) to fig.set_layout_engine('tight') (newly deprecated in Matplotlib).
• Update unit tests using deprecated functionality to use current functionality.
• Add warning filters for internal calls to connect (which are OK for now).
• Avoid calling tf2zpk with zero in the numerator (SciPy generates warning).
• Fix up some strings that had newly deprecated escape sequences (by using raw strings)
• Turn off warnings for optimal control problems that fail to hit desired precision (but still return valid answers)

Note: it is likely that this PR will conflict with #953. We should probably merge #953 first and then rebase this PR since the changes here are pretty straightforward.

[coveralls]

coverage: 94.78% (-0.09%) from 94.866%
when pulling dc74493 on murrayrm:fixwarnings-23Dec2023
into eb0f3f9 on python-control:main.

[bnavigator]

Note: it is likely that this PR will conflict with #953. We should probably merge #953 first and then rebase this PR since the changes here are pretty straightforward.

Either this or the other way round. Python 3.12 requires numpy 1.26 on conda, already triggering the deprecation warnings. I had to disable them for the Slycot CI: https://github.com/python-control/Slycot/pull/227/files#diff-b2cf1d5b6cad7a0f6a749da416cb8a4e34040b2164a84a651783692d4b60d608R17

[murrayrm]

Looks like there were no conflicts, so going ahead and merging.

@bnavigator: FYI (and not really for this PR), I tried updating GitHub actions to use Python 3.12 but mamba caused problems, so I'm waiting until that clears. You can find the attempt here.