setting `exclude_gitignore = true` breaks stubtest

[jorenham]

When I add exclude_gitignore = true to the [tool.mypy] in the pyproject.toml of numtype, stubtest suddenly fails to find the numpy-stubs, while mypy itself still seems to works. This also occurs with an empty .gitignore.

Note that stubtest runs in an isolated uv environment (tool/stubtest.py), because without it, it doesn't work at all 🤷🏻.

See https://github.com/numpy/numtype/pull/584 for the repro and full stubtest output.