Mandelbrot example / NumPy DeprecationWarning

[PierreRaybaut]

From Issue #98:

I found additional incompatibilities. In the 'mandelbrot' test, the following warning is raised:

guiqwt\tests\mandelbrot.py:60: DeprecationWarning: NumPy will stop allowing conversion of out-of-bound Python integers to integer arrays.  The conversion of -1 to uint8 will fail in the future.
For the old behavior, usually:
    np.array(value).astype(dtype)`
will give the desired result (the cast overflows).