The docstring parsing in get_aliases was brittle and didn't handle the
final dot that cbook._define_aliases added to the docstring, so as of
master it would return the keys with an additional dot. Instead, use a
regex which will loudly error out if the format is incorrect.

Also replace {key: None, ...} mapping simulating a set to, well, a set.
(The use of sets instead of lists was added in ed9c2b5 apparently to
handle the case where the same alias is added twice.)

(Yes, the whole alias machinery could now be simplified e.g. by adding
an is_mpl_alias attribute on the aliases instead.)

At least the parsing fix should be considered RC as the issue came in in
#9475.

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is Flake 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