* so that excess files are not included in sdist, perform build down
in circuitpython-stubs
* This means we need to
* Remove the need-pypi check
* Copy a setup.py, README, and MANIFEST.in into the stubs build location
* Revamp how the overall `mypy --strict` check lists its inputs
* Add a new test that actually installing the stubs lets us do type
checking (tools/test-stubs.sh)
* Add a missing return type to a __init__ function (why was this not
an error under `mypy --strict`, I wonder)
