permits black to run in AWS Lambda: (#1141)

AWS Lambda and some other virtualized environment may not permit access
to /dev/shm on Linux and as such, trying to use ProcessPoolExecutor will
fail.

As using parallelism is only a 'nice to have' feature of black, if it fails
we gracefully fallback to a monoprocess implementation, which permits black
to finish normally.

Co-authored-by: Allan Simon <asimon@yolaw.fr>