Modernize/improve various development things

I noticed a couple of things while picking up work on pytest-qt again:

• One test uses tmpdir instead of tmp_path
• All tests use testdir instead of pytester
• We might want to switch from autoflake/pyupgrade/black/flake8 to ruff?
• We should switch from setup.py to a pyproject.toml (and maybe uv?)
• License PyPI classifiers are deprected, we should use a SPDX license
• We should force colored pytest output on GitHub Actions
• We should have type annotations and use mypy on CI (and then probably add py.typed too)

Thoughts? We should probably split this into more bite-sized issues, just writing down what went through my head before I forget everything again.

Start by splitting the checklist into focused issues, then inspect the tests using tmpdir/testdir and the project’s setup.py and GitHub Actions configuration. Review the proposed pyproject.toml, tooling, licensing, output, and typing changes separately; done should mean each scoped modernization has an agreed implementation and passing CI.