Declare plugin is compatible with parallel reading

[blaise-io]

I'm getting a warning from this plugin, which is problematic because I want to treat warnings as errors:

the humanfriendly.sphinx 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

See https://www.sphinx-doc.org/en/master/extdev/index.html#extension-metadata

parallel_write_safe can be omitted (tested locally).

[xolox]

Thanks Blaise, this was merged and released as part of humanfriendly 9.2.