Deprecation warning from Sympy

There's a deprecation warning coming from sympy, related to the use of None in Matrixes. I did not attempt to solve this, as it means digging deeper into the functionality of the library.

The specific warning is:

SymPyDeprecationWarning: 

non-Expr objects in a Matrix is deprecated. Matrix represents
a mathematical matrix. To represent a container of non-numeric
entities, Use a list of lists, TableForm, NumPy array, or some
other data structure instead.

During doctest, the warning appears twice. Here are the occurrences:

• ode_system.py
  • ode_system.py:666
  • self._derivatives = tuple(sympy.Matrix(self._derivatives).extract(column_shuffle, [0]))
• matrix_normal_forms.py
  • matrix_normal_forms.py:496
  • return sympy.Matrix(matrix_.shape[0], matrix_.shape[1], lambda i, j: matrix_[i, j] < other)