Chartify

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* Relaxed scipy and pandas version requirements to allow verions 2.x

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* Documentation build fix
* Pin tornado requirement to reduce vulnerability

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* Require jupyter_bokeh to enable html output

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* Fix categorical_order_by check for scatter plot
* Fix categorical_order_by check for _construct_source
* Refactor category sorting in _construct_source
* Add tests for categorical_order_by
* Fix scatter plot tests that used line plots

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* Updated version requirement of pillow to avoid bug

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* Dropped support for python 3.6 and 3.7