Set python version for mypy

It looks like `mypy` started to check everything assuming the interpreter python version, not the project python version. Could we please add this block to `pyproject.toml`:
```toml
[tool.mypy]
python_version = "{min_python_version}"
```
where `min_python_version` is e.g. 3.10.

It was tested with `nested-pandas`:
https://github.com/lincc-frameworks/nested-pandas/pull/303