In mpl_toolkits, use the same floating point slop as for standard ticks.

[anntzer]

Standard ticks use _interval_contains_close (in Axis._update_ticks); do
the same in mpl_toolkits and deprecate the custom slops delta1, delta2
(if such knobs are really needed, we should try to have the same API in
the main library anyways).

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).