Explicitly tell sphinx that theme is parallel safe

[rikardn]

Setting the parallel_read_safe sphinx metadata key would allow for faster builds using this theme.

It would also avoid the following warning when building documentation using the theme:

WARNING: the pydata_sphinx_theme 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 explic
it
WARNING: doing serial read

I have tested the new settings on the documentation on my own project and I cannot see any problems.

[choldgraf]

this seems pretty reasonable to me - maybe @jorisvandenbossche can confirm that he also agrees it is parallel read/write safe?

Can you fix the linting bug (my guess is that it comes with not black-ifying your change)

[jorisvandenbossche]

Yes, I don't directly see a reason why it wouldn't be parallel safe, so this sounds good!

[rikardn]

Thanks for considering this! I amended the fix for the linter to the same commit

[choldgraf]

thanks very much @rikardn !