Feature request: permits to disable parallel run for system without support for it

[allan-simon]

On some virtualized environment you don't have access to /dev/shm which prevent the use of multi-processing. The most notable use case being AWS Lambda, where /dev/shm is simply not present.

A quick and dirty patch show that simply replacing https://github.com/psf/black/blob/master/black.py#L544 by None
and removing this line https://github.com/psf/black/blob/master/black.py#L559
permits black to run in AWS Lambda, which is a huge win in term of CI (as black fits perfectly the use for serverless, i.e short runtime, low memory consumption, triggered on specific event)

so it could be implemented either (or both) by providing a --no-parallel or by detecting that /dev/shm is not possible (I need to dig on how to do that) and in these case to not use the ProcessPoolExecutor

I can provide the pull request if it seems acceptable to you

[allan-simon]

I've added a PR