* Enable maths in singlehtml mode

Setting the html_assets_policy flag  to always installs mathjax in singlehtml builder. This also helps in maths rendering for pdfhtml builder.

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci



Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>