Fix deprecation warning

[johannespitz]

.../urdfpy/urdf.py:898: 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
  value = np.asanyarray(value).astype(np.float)

I was getting this deprecation warning quite often, so I decided to fix it.

[johannespitz]

I also searched through the repository, and couldn't find another occurrence of "np.float" (without the 64 following it).