Extension is safe for parallel reading, please mark it as such

[mjpieters]

When building documentation:

WARNING: the sphinx_gitref 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
WARNING: doing serial read

You'd only set "parallel_read_safe": False if an extension needs to have access to shared information while source files are read and parsed. Or, put differently, if it were to break when run in multiprocessing jobs. I don't see anything in the codebase that would cause such issues.

Please mark the extension is safe for parallel reads by including "parallel_read_safe": True in the dictionary returned from setup(), see https://www.sphinx-doc.org/en/1.3.1/extdev/index.html#extension-metadata

I've locally added the change with no issues:

    return {"version": __version__, "parallel_read_safe": True}