Fix numpy indexing issue

[matsievskiysv]

On Python 3.11.2 with numpy 1.24.3 I get the following indexing error:

  File "/mnt/mstorage/Projects/Python/beadpull/.venv/lib/python3.11/site-packages/smithplot/smithaxes.py", line 1070, in grid
    for ys in np.linspace(y0, y1, y_div + 1)[1:]:
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<__array_function__ internals>", line 200, in linspace
  File "/mnt/mstorage/Projects/Python/beadpull/.venv/lib/python3.11/site-packages/numpy/core/function_base.py", line 121, in linspace
    num = operator.index(num)
          ^^^^^^^^^^^^^^^^^^^
TypeError: 'numpy.float64' object cannot be interpreted as an integer

This PR fixes this error.