Fix deprecation warning in simple_linear_interpolation

On master, when drawing polar plots:

    cbook.py:1744: DeprecationWarning: using a non-integer number instead of an 
    integer will result in an error in the future
        result = np.zeros(new_shape, a.dtype)

Ps: could I have pushed this on #2100 instead of creating another PR ?
should not need the float as we have `from __future__ import divsion`