Pytest skeleton in mixed template

[nils-werner]

I believe it's a good practice to nudge people towards best practices, and having unit tests is definitely a best practice that should be embraced.

This PR adds a small Pytest skeleton to projects created with maturin init --mixed, so that people know where to place their Python unit tests, and are nudged towards writing them. The resulting structure is as follows:

├── Cargo.toml
├── pyproject.toml
├── python
│   ├── asd
│   │   └── __init__.py
│   └── tests
│       └── test_all.py
└── src
    └── lib.rs

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