The setup() function of a Sphinx-extension can return a dictionary. This
is treated by Sphinx as metadata of the extension [1].
With metadata "parallel_read_safe = True" a extension is marked as
save for "parallel reading of source". This is needed if you want
build in parallel with N processes. E.g.:
make SPHINXOPTS=-j4 htmldocs
will no longer log warnings like:
WARNING: the foobar 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.