Don't sort lexicographially entries in SVG output.

[anntzer]

Sorting had been added in cb5893d for reproducible outputs, but dicts
now maintain insertion order so this is not needed anymore; moreover,
not sorting allows keeping attributes in a more semantic meaning --
e.g., attributes of the toplevel <svg> are now in the order
xmlns:xlink width height viewBox xmlns version rather than
height version viewBox width xmlns xmlns:xlink; the <use> elements
of ticks now have attributes xlink:href x y style rather than
style x xlink:href y.

The order is still not perfect due to other issues, but that'll be
addressed another time.

PR Summary

PR Checklist

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (run flake8 on changed files to check).
• New features are documented, with examples if plot related.
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• Conforms to Matplotlib style conventions (install flake8-docstrings and run flake8 --docstring-convention=all).
• New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).