Support linting when missing sem_open syscall
Platforms such as Termux on Android, and other exotic devices do not provide a sem_open implementation on the OS level. This is problematic, as the error resulting from this occurs when calling multiprocessing.Pool, throwing an unhandled ImportError.
The issue itself is outlined in https://bugs.python.org/issue3770.
This change allows devices missing this system call to respond to the missing feature by falling back to synchronous execution, which appears to be the default behaviour if the multiprocessing module is not found.
This change also adds a potential fix for developers working on platforms where multiprocessing itself cannot be imported. The existing code would set the name referencing the import to None, but there are no clear checks to ensure this does not result in an AttributeError later when multiprocessing.Pool has accession attempts.
Existing users should see no difference in functionality, as they will assumably already be able to use flake8, so will not be missing this sem_open call.
Users on devices without the sem_open call will now be able to use flake8 where they would be unable to before due to unhandled ImportErrors.