REL: v3.4.0

Highlights of this release include:

- Figure and Axes creation / management
    - New subfigure functionality
    - Single-line string notation for subplot_mosaic
    - Changes to behavior of Axes creation methods (gca, add_axes,
      add_subplot)
    - add_subplot/add_axes gained an axes_class parameter
    - Subplot and subplot2grid can now work with constrained layout
- Plotting methods
    - axline supports transform parameter
    - New automatic labeling for bar charts
    - A list of hatches can be specified to bar and barh
    - Setting BarContainer orientation
    - Contour plots now default to using ScalarFormatter
    - Axes.errorbar cycles non-color properties correctly
    - errorbar errorevery parameter matches markevery
    - hexbin supports data reference for C parameter
    - Support callable for formatting of Sankey labels
    - Axes.spines access shortcuts
    - New stairs method and StepPatch artist
    - Added orientation parameter for stem plots
    - Angles on Bracket arrow styles
    - TickedStroke patheffect
- Colors and colormaps
    - Collection color specification and mapping
    - Transparency (alpha) can be set as an array in collections
    - pcolormesh has improved transparency handling by enabling snapping
    - IPython representations for Colormap objects
    - Colormap.set_extremes and Colormap.with_extremes
    - Get under/over/bad colors of Colormap objects
    - New cm.unregister_cmap function
    - New CenteredNorm for symmetrical data around a center
    - New FuncNorm for arbitrary normalizations
    - GridSpec-based colorbars can now be positioned above or to the
      left of the main axes
- Titles, ticks, and labels
    - supxlabel and supylabel
    - Shared-axes subplots tick label visibility is now correct for top
      or left labels
    - An iterable object with labels can be passed to Axes.plot
- Fonts and Text
    - Text transform can rotate text direction
    - matplotlib.mathtext now supports overset and underset LaTeX
      symbols
    - math_fontfamily parameter to change Text font family
    - TextArea/AnchoredText support horizontalalignment
    - PDF supports URLs on Text artists
- rcParams improvements
    - New rcParams for dates: set converter and whether to use
      interval_multiples
    - Date formatters now respect usetex rcParam
    - Setting image.cmap to a Colormap
    - Tick and tick label colors can be set independently using rcParams
- 3D Axes improvements
    - Errorbar method in 3D Axes
    - Stem plots in 3D Axes
    - 3D Collection properties are now modifiable
    - Panning in 3D Axes
- Interactive tool improvements
    - New RangeSlider widget
    - Sliders can now snap to arbitrary values
    - Pausing and Resuming Animations
- Sphinx extensions
    - plot_directive caption option
- Backend-specific improvements
    - Consecutive rasterized draws now merged
    - Support raw/rgba frame format in FFMpegFileWriter
    - nbAgg/WebAgg support middle-click and double-click
    - nbAgg support binary communication
    - Indexed color for PNG images in PDF files when possible
    - Improved font subsettings in PDF/PS
    - Kerning added to strings in PDFs
    - Fully-fractional HiDPI in QtAgg
    - wxAgg supports fullscreen toggle