Inheritable pytest configuration

Hi, thanks for making pytest!

I'm trying out a monorepo setup which has a pyproject.toml file at the root level and one per sub-project in the monorepo. Ideally, I'd like to set something like

# ./pyproject.toml
[tool.pytest.ini_options]
asyncio_mode = "auto"

only once in the root-level pyproject.toml file and the sub-projects automatically inherits that setting unless overridden in their respective pyproject.toml. Another possibility would be to do it like ruff (see https://docs.astral.sh/ruff/settings/#extend). So something like this in the sub-project's pyproject.toml

# ./sub_project/pyproject.toml
[tool.pytest.ini_options]
extend = "../pyproject.toml"
# Inherits asyncio_mode = "auto"
addopts = "-v"

Cheers!

Start with the root and sub-project pyproject.toml examples in the issue and trace how pytest discovers configuration for each project. Define and verify inheritance from the root configuration, including the proposed extend form, while preserving sub-project overrides; done means the requested settings work across the monorepo without duplication.