Does not make parallel build support explicit

[t-b]

When doing a parallel build with sphinx-build -j auto ... I get

WARNING: the sphinxcontrib.fulltoc extension does not declare if it is safe for parallel reading,
assuming it isn't - please ask the extension author to check and make it explicit

This is with sphinx 4.4. Does fulltoc support parallel builds?

[danepowell]

Seems to work: https://github.com/sphinx-contrib/fulltoc/pull/26