This warning is produced when running tests with Matplotlib 3.0, see, e.g., https://travis-ci.org/python-control/python-control/jobs/473223941

The warning is
MatplotlibDeprecationWarning: The find function was deprecated in Matplotlib 2.2 and will be removed in 3.1.

I can't find info on when Matplotlib 3.1 is likely to be released.

https://docs.scipy.org/doc/numpy-1.13.0/user/numpy-for-matlab-users.html#linear-algebra-equivalents says that numpy.nonzero is equivalent to Matlab's find, so perhaps this isn't too difficult to fix.