Allow dmypy to be run on v3 branch

[dstansby]

This allows dmypy, the mypy daemon to be run on the v3 branch. In turn this allows the mypy plugin to be used in VSCode.

follow_imports is not supported by the daemon, but not needed, and namespace_packages = false is needed so mypy doesn't interpret src/ as a package.

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/tutorial.rst
• Changes documented in docs/release.rst
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)