REL: v3.7.0

Highlights of this release include:

- Plotting and Annotation improvements
  - ``hatch`` parameter for pie
  - Polar plot errors drawn in polar coordinates
  - Additional format string options in `~matplotlib.axes.Axes.bar_label`
  - ``ellipse`` boxstyle option for annotations
  - The *extent* of ``imshow`` can now be expressed with units
  - Reversed order of legend entries
  - ``pcolormesh`` accepts RGB(A) colors
  - View current appearance settings for ticks, tick labels, and gridlines
  - Style files can be imported from third-party packages
- Improvements to 3D Plotting
  - 3D plot pan and zoom buttons
  - *adjustable* keyword argument for setting equal aspect ratios in 3D
  - ``Poly3DCollection`` supports shading
  - rcParam for 3D pane color
- Figure and Axes Layout
  - ``colorbar`` now has a *location* keyword argument
  - Figure legends can be placed outside figures using constrained_layout
  - Per-subplot keyword arguments  in ``subplot_mosaic``
  - ``subplot_mosaic`` no longer provisional
- Widget Improvements
  - Custom styling of button widgets
  - Blitting in Button widgets
- Other Improvements
  - Source links can be shown or hidden for each Sphinx plot directive
  - Figure hooks
- New & Improved Narrative Documentation
  - Brand new :doc:`Animations </tutorials/introductory/animation_tutorial>` tutorial.
  - New grouped and stacked `bar chart <../../gallery/index.html#lines_bars_and_markers>`_ examples.
  - New section for new contributors and reorganized git instructions in the :ref:`contributing guide<contributing>`.
  - Restructured :doc:`/tutorials/text/annotations` tutorial.