CMake fails to find Python library for tests

I am building spglib v2.1.0 with CMake on Debian unstable:
```bash
mkdir build && cd build
cmake .. -DSPGLIB_WITH_Fortran=ON -DSPGLIB_WITH_Python=ON
make
make test
```
Tests fail for python-tests with `ModuleNotFoundError: No module named 'spglib'`. I tried setting `PYTHONPATH` to various locations seemingly containing Python files after the build, but I did not succeed.