muscle3 cannot import libmuscle.planner in editable mode install

[DaanVanVugt]

When creating an editable mode install of muscle3 (following the guide at https://muscle3.readthedocs.io/en/latest/contributing.html) we get a problem running the muscle3 command:

File “/Users/daanvanvugt/muscle3/muscle3/muscle3.py”, line 11, in <module>
  from libmuscle.planner.planner import (
ModuleNotFoundError: No module named ‘libmuscle.planner’

It seems to be related to the editable mode install (perhaps because the folder structure for libmuscle is different, containing also a python folder).

[DaanVanVugt]

duplicate of #102