PR Summary

According to#20720 (comment), compressing images takes about 15% of the build time, as does the uncollapsed navigation, so don't do those costly steps if running Circle CI.

Note that the pydata-sphinx-theme is still quite slow, taking 50% more time than the bare build without the other two options.

PR Checklist

• Has pytest style unit tests (andpytest passes).
• IsFlake 8 compliant (runflake8 on changed files to check).
• New features are documented, with examples if plot related.
• Documentation is sphinx and numpydoc compliant (the docs shouldbuild without error).
• Conforms to Matplotlib style conventions (installflake8-docstrings and runflake8 --docstring-convention=all).
• New features have an entry indoc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented indoc/api/next_api_changes/ (follow instructions in README.rst there).