e.g. from element.py:

   from . import SandboxFlags # This works
   from . import Directory, FileBasedDirectory, VirtualDirectoryError # This doesn't

sandbox/__init__.py:

   from .sandbox import Sandbox, SandboxFlags
   from ._sandboxchroot import SandboxChroot
   from ._sandboxbwrap import SandboxBwrap

storage/__init__.py:

   from .Directory import Directory
   from ._filebaseddirectory import FileBasedDirectory, VirtualDirectoryError

./__init__.py:
    from .sandbox import Sandbox, SandboxFlags
    from .storage import Directory, FileBasedDirectory, VirtualDirectoryError