Remove string escapes from matplotlibrc.template.
This fixes #10406. Since the matplotlib rc file is not a python file, the rc arguments should not contain string escape characters. E.g. instead of formerly

boxplot.medianprops.color : 'C1'

it should read

boxplot.medianprops.color : C1

This change makes it easier to just uncomment the lines in the template file for use in a real rc file.

Note that I left some of the arguments in the "animations" section unchanged, because it is not clear what to put there instead. In any case, the fixed version of the template allows to uncomment any rc line (including those from the animations section) and not get a warning, so I suppose the animation parameters are correct, although they contain string escapes.

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is PEP 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way