Small fix to Histogram2dContour setup in plotly express

[gmjw]

As per the docs, if z is passed in with no histfunc, it should be set to 'sum':

I think this corrects the intended behaviour and puts Histogram2dContour more in line with Histogram2d.

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[gmjw]

If a more experienced contributor takes a look at this - I can't understand why the Circle CI build has failed, the error message doesn't indicate anything clearly - any advice appreciated.

[gvwilson]

thanks @gmjw - much appreciated. the broken Circle CI job isn't your fault - we're working on it…

[emilykl]

@gmjw This looks great, thanks for the fix. The broken CI job has been fixed on main; it should pass if you merge main into this branch.