Error when building docs in parallel

[dralley]

When using sphinx-build -j auto:

the sphinxcontrib.httpdomain 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

[humitos]

Do you know if the extension is parallel_read_safe=True? If so, I can do PR to set it.

Reference https://www.sphinx-doc.org/en/master/extdev/#extension-metadata

[akonradi]

I don't see why it wouldn't be safe to read in parallel

[phlax]

Seems like this may have been resolved in #31