Doesn't work for afpfs

[3h4x]

Hey
For lock file in network attached afpfs lock was created for every python process I ran and didn't prevent parallel execution.
I expected to receive a warning or error if it doesn't work.

For local lock file everything works fine.

Could be added to readme that it's incompatible.
I'm not that much familiar with afpfs and code of this project.

[gaborbernat]

Hello, if you make a PR for this (with tests) we would be happy to review it, thanks!