update np.float to float to fix NumPy 1.20 deprecation warnings

[murrayrm]

Fixes the following warning (and others like it):

  /Users/murray/Dropbox/macosx/src/python-control/murrayrm/control/tests/timeresp_test.py:411: DeprecationWarning: `np.float` is a deprecated alias for the builtin `float`. To silence this warning, use `float` by itself. Doing this will not modify any behavior and is safe. If you specifically wanted the numpy scalar type, use `np.float64` here.
  Deprecated in NumPy 1.20; for more details and guidance: https://numpy.org/devdocs/release/1.20.0-notes.html#deprecations

[coveralls]

Coverage remained the same at 88.79% when pulling f35331b on murrayrm:scipy_1_20_deprecations into 4788f6b on python-control:master.