New setup.py causes incomplete installation

The new setup process looks much cleaner now, but sadly it causes an incomplete installation for me.

Steps to reproduce

1. Create fresh virtualenv
2. Run pip install .

Result

> find $VENV/lib/python3.9/site-packages/tkwant
  $VENV/lib/python3.9/site-packages/tkwant
  $VENV/lib/python3.9/site-packages/tkwant/onebody
  $VENV/lib/python3.9/site-packages/tkwant/onebody/kernels.cpython-39-x86_64-linux-gnu.so
  $VENV/lib/python3.9/site-packages/tkwant/onebody/solvers.cpython-39-x86_64-linux-gnu.so