Import Python submodules crashes Julia

I have found this issue when importing python submodules with `algotom`

The following results in segmentation fault:

```julia
pyimport("algotom.prep.removal")
```

On the contrary, importing only, say, `pyimport("algotom.prep")` works fine.

Start by reproducing the Julia snippet with pyimport("algotom.prep.removal"), then compare it with the working pyimport("algotom.prep") case. Investigate the Python submodule import boundary in PythonCall.jl; done means the submodule import no longer segfaults while the parent-module import continues to work.

索引モデルが issue の本文から書いたものです。