Swap mypy in favour of ty

[paddyroddy]

ty is a fast alternative to mypy. Although it has been released for a while now, the pre-commit hook has only just been released. We've used this successfully in GLASS glass-dev/glass#1133 and I've used it in SLEPLET astro-informatics/sleplet#489. It is being rapidly developed so will only get better. I know Astral sold out, but I doubt we're going to move from uv realistically.

[paddyroddy]

I am using https://github.com/j178/prek in favour of pre-commit locally. I found that running on the top level was running the hooks in tests/data/test_package_generation which was causing bugs.

[paddyroddy]

@pre-commit.ci seems like it's having a mare

error: Request failed after 3 retries in 6.8s
  Caused by: Failed to fetch: `https://pypi.org/simple/pytest-venv/`
  Caused by: error sending request for url (https://pypi.org/simple/pytest-venv/)
  Caused by: client error (Connect)
  Caused by: dns error
  Caused by: failed to lookup address information: Temporary failure in name resolution

[Copilot]

Pull request overview

This PR updates the cookiecutter template (and its test fixture) to use Astral’s ty type checker instead of mypy, and wires ty into pre-commit so type checking runs consistently in local development and CI.

Changes:

• Replace the mirrors-mypy pre-commit hook with astral-sh/ty-pre-commit across the root config, template, and test fixture.
• Add dependency-groups.typechecking and tool.ty configuration to support isolated type-check runs.
• Update documentation to recommend ty (while still listing mypy as an alternative).

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
tests/data/test_package_generation/pyproject.toml	Updates fixture dependencies/config from mypy to ty and adds typechecking group/config.
tests/data/test_package_generation/.pre-commit-config.yaml	Switches fixture pre-commit type checking hook from mypy to ty.
pyproject.toml	Adds a typechecking dependency group and ty overrides for this repository.
docs/pages/tutorial.md	Updates recommended tooling to reference ty instead of mypy.
docs/pages/linting.md	Promotes ty as the preferred type checker and downgrades mypy to “Good”.
docs/pages/libraries/jupyter-notebooks.md	Updates notebook linting guidance to reference ty instead of mypy.
{{cookiecutter.project_slug}}/pyproject.toml	Updates template dev dependencies/config from mypy to ty and adds typechecking group/config.
{{cookiecutter.project_slug}}/.pre-commit-config.yaml	Switches template pre-commit type checking hook from mypy to ty.
.prekignore	Adds ignore rules for prek users (separate from pre-commit’s own excludes).
.pre-commit-config.yaml	Switches this repo’s pre-commit type checking hook from mypy to ty.

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