errorfill incompatible with newer matplotlib versions

[loichuder]

Hi there,

Wanting to use the spectral.errorfill function that I find quite handy, I got this error:

"...mpltools/special/errorfill.py" line 44, in errorfill:
    color = next(ax._get_lines.color_cycle)
AttributeError: '_process_plot_var_args' object has no attribute 'color_cycle'

with matplotlib 3.1.1.

In fact, color_cycle was deprecated in matplotlib 2.2.0: https://matplotlib.org/api/prev_api_changes/api_changes_2.2.0.html#id1
The solution is to use the new prop_cycle (I managed to have it working this way).

I do not know if this is still maintained but I could drop a PR.