Adding functionality to recognise Darwin as a platform in plaform.py

non-Linux platforms don't return EISDIR when attempting to unlink a directory

Stopped safe_remove attempting to unlink dirs
Previously safe_remove would attempt to unlink a path
Before attempting to remove it if it was a dir

Now it checks for a dir before that step

Sandbox: Ensure that we only import the sandbox subclasses when instantiate a sandbox

This patch was required due to not being able to execute `bst` on
a Linux machine without Fuse or Bubblewrap available