Mpl-scatter-density

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- Fix compatibility with Matplotlib 3.3. [30]

- Drop support for Python 2.7 and require Matplotlib>=3. [30]

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- Avoid unecessary imports of Qt and Tk. [27]

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- Internal code reorganization to faciliate re-use in other
packages. [26]

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- Added a keyword argment ``update_while_panning`` to set whether to
compute the scatter denstiy map while panning and zooming. [16]

- Downsample when resizing interactive window. [14]

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- Added support for the ``c=`` argument, similar to the argument with
the same name in Matplotlib's ``scatter`` function. [11]

- Added support for log axes. [11]

- Fixed support for flipped limits (e.g. xmax < xmin). [11]

- Added support for setting vmin/vmax to functions to determine limits
on-the-fly. [11]

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- Fix compatibility with the Jupyter notebook. [8]