build: avoid hard-coding mypy deps in pre-commit

Currently we hard code python dependencies in the pre-commit config to run mypy with the project's dependencies. This is obviously not ideal from a maintainability point of view.

I did some investigation into improving this situation, and I've found that we can run poetry from a python venv pre-commit manages and still have it act on the poetry's own normal venv for the project dir. We can do this by un-setting the VIRTUAL_ENV environment variable when running poetry; this results in poetry ignoring the pre-commit venv it's installed in, and instead using its normal project venv detection logic to use the project dir's poetry venv.

As a result, if we use this method to install poetry dependencies via poetry's pre-commit hook and use it to run the typecheck task from pyproject.toml, poetry will run the mypy version from the poetry-managed project venv (which is also the developer's own project venv when running on a dev machine, which speeds up pre-commit hook runs an reduces disk space usage).

This avoids the need to hardcode all the dependencies and versions, as the pre-commit typecheck task will automatically use poetry's deps.

I've also changed the pre-commit CI job to run the post-checkout pre-commit stage, because Poetry's install hook triggers on checkout, not commit, and this isn't run in CI without this manual intervention.