Address multiple deprecation warnings emerging at Python 3.12

The test suite is showing that there are at last a few deprecation warnings emerging from standard cf-plot code, which therefore probably needs to be updated accordingly in its use of numpy, matplotlib and perhaps other libraries too:

...
/home/slb93/miniconda3/envs/cf-env-312/lib/python3.12/site-packages/numpy/ma/core.py:5074: DeprecationWarning: Calling nonzero on 0d arrays is deprecated, as it behaves surprisingly. Use `atleast_1d(cond).nonzero()` if the old behavior was intended. If the context of this warning is of the form `arr[nonzero(cond)]`, just use `arr[cond]`.
  return narray(self.filled(0), copy=False).nonzero()
...
/home/slb93/miniconda3/envs/cf-env-312/lib/python3.12/site-packages/matplotlib/cbook.py:1699: DeprecationWarning: Conversion of an array with ndim > 0 to a scalar is deprecated, and will error in future. Ensure you extract a single element from your array before performing this operation. (Deprecated NumPy 1.25.)
  return math.isfinite(val)
...

We should make updates to prevent any DeprecationWarning emerging.