REL: v3.9.0

Highlights of this release include:

- Plotting and Annotation improvements
   - Axes.inset_axes is no longer experimental
   - Legend support for Boxplot
   - Percent sign in pie labels auto-escaped with usetex=True
   - hatch parameter for stackplot
   - Add option to plot only one half of violin plot
   - axhline and axhspan on polar axes
   - Subplot titles can now be automatically aligned
   - axisartist can now be used together with standard Formatters
   - Toggle minorticks on Axis
   - StrMethodFormatter now respects axes.unicode_minus
- Figure, Axes, and Legend Layout
   - Subfigures now have controllable zorders
   - Getters for xmargin, ymargin and zmargin
- Mathtext improvements
   - mathtext documentation improvements
   - mathtext spacing corrections
- Widget Improvements
   - Check and Radio Button widgets support clearing
- 3D plotting improvements
   - Setting 3D axis limits now set the limits exactly
- Other improvements
   - New BackendRegistry for plotting backends
   - Add widths, heights and angles setter to EllipseCollection
   - image.interpolation_stage rcParam
   - Arrow patch position is now modifiable
   - NonUniformImage now has mouseover support