Directly manipulating a list of child axes is strange compared to the
standard Matplotlib axes creation API, so don't use that in mpl_toolkits
parasite axes either. Instead, use the already existing get_aux_axes
API. However, that API needs to be improved to support arbitrary
kwargs that get forwarded to the Axes constructor, which is easy to do.

Also replace the default axes_class in that API by _base_axes_class,
which is consistent with HostAxes.twin()/twinx()/twiny(), and also
corresponds to the practical use cases demonstrated in the examples: if
the host axes class is from axisartist, the parasite axes class is also
from axisartist (see examples/axisartist/demo_parasite_axes.py); using
another parasite axes class won't work.

This is preliminary work for making parasite_axes a thin wrapper around
standard child_axes.

PR Summary

PR Checklist

Tests and Styling

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (install flake8-docstrings and run flake8 --docstring-convention=all).

Documentation

• New features are documented, with examples if plot related.
• 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).
• Documentation is sphinx and numpydoc compliant (the docs should build without error).