Highlights of this release include:

- Provisional API for composing semantic axes layouts from text or
  nested lists
- New Axes.sharex, Axes.sharey methods
- Turbo colormap
- colors.BoundaryNorm supports extend keyword argument
- Text color for legend labels
- Pcolor and Pcolormesh now accept shading='nearest' and 'auto'
- Allow tick formatters to be set with str or function inputs
- New Axes.axline method
- Dates use a modern epoch
- Improved font weight detection
- Axes3D no longer distorts the 3D plot to match the 2D aspect ratio
- More consistent toolbar behavior across backends
- Toolbar icons are now styled for dark themes
- Cursor text now uses a number of significant digits matching pointing
  precision
- Functions to compute a Path's size
- savefig() gained a backend keyword argument
- Saving SVG now supports adding metadata
- Saving PDF metadata via PGF now consistent with PDF backend
- NbAgg and WebAgg no longer use jQuery & jQuery UI

For the full details please see the whats new [1] and API changes [2]
in the documentation

[1] https://matplotlib.org/3.3.0/users/whats_new.html
[2] https://matplotlib.org/3.3.0/api/api_changes.html