Bug with `bartlett` window in spectrogram

Created by: paulaltin

There seems to be a bug when using the bartlett window to make a spectrogram. When I run the example on this page: https://gwpy.github.io/docs/v0.1/examples/spectrogram/plot.html with method='bartlett' (and without an overlap argument), it returns TypeError: welch() got multiple values for keyword argument 'noverlap'.

​I think this can be fixed by adding kwargs.pop('noverlap', None) to line 57 of spectrum/scipy_.py.