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.

• [x] I have read through the contributing notes and understand the structure of the package. In particular, if my PR modifies code of plotly.graph_objects, my modifications concern the code generator and not the generated files.
• [ ] I have added tests or modified existing tests.
• [ ] For a new feature, I have added documentation examples (please see the doc checklist as well).
• [ ] I have added a CHANGELOG entry if changing anything substantial.
• [ ] For a new feature or a change in behavior, I have updated the relevant docstrings in the code.

[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.