FigureCanvasWebAgg and FigureCanvasNbAgg empty subclasses of
FigureCanvasWebAggCore. They only differed in that WebAggCore did not
define the (common) timer class, which is easily fixed (but the
TimerTornado definition needs to be moved above the
FigureCanvasWegAggCore definition), and that WebAgg explicitly redefined
show to use the module'sshow, whereas WebAggCore'sshow uses
pyplot.show... butpyplot.show is defined as calling the backend
module'sshow, so it comes down to the same.

Still it's useful for them to be subclasses rather than straight
aliases, to keep the possibility of a canvas->manager mapping (#18854).

No need to explicitly mark WebAggCore assupports_blit, as that's
autodetected now (one can easily check that
FigureCanvasWebAggCore.supports_blit is still True).

Savingpgf to a BytesIO actually works fine nowadays, and can indeed
by manually tested on WebAgg.

PR Summary

PR Checklist

• Has pytest style unit tests (andpytest passes).
• IsFlake 8 compliant (runflake8 on changed files to check).
• New features are documented, with examples if plot related.
• Documentation is sphinx and numpydoc compliant (the docs shouldbuild without error).
• Conforms to Matplotlib style conventions (installflake8-docstrings and runflake8 --docstring-convention=all).
• New features have an entry indoc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented indoc/api/next_api_changes/ (follow instructions in README.rst there).