Enable parallel write

[marscher]

in #9 parallel reading was enabled. Now lets do the same for writing. I've tested this with my (large) project locally. The output looks sane and the total build time was cut in half.

[gaborbernat]

Can we add a changelog entry for this?

[hoodmane]

Sphinx docs say:

'parallel_write_safe': a boolean that specifies if parallel writing of output files can be used when the extension is loaded. Since extensions usually don’t negatively influence the process, this defaults to True.

If it defaults to true, then this shouldn't have any effect. Is that incorrect?

[marscher]

From my observation it does not, otherwise I could not explain why the build time decreases to 50%. This was taken with Sphinx 5.3

[hoodmane]

Well the change is either harmless or useful so it isn't a big deal to merge it. But the code for v5.3.0 also seems to show that this defaults True:

         self.parallel_write_safe = kwargs.pop('parallel_write_safe', True)

https://github.com/sphinx-doc/sphinx/blob/v5.3.0/sphinx/extension.py#L33

used here:
https://github.com/sphinx-doc/sphinx/blob/v5.3.0/sphinx/application.py?plain=1#L1278