Documentation fails to mention installation of test dependencies

[Xiretza]

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When following the documentation on https://developers.home-assistant.io/docs/development_environment and https://developers.home-assistant.io/docs/development_testing, pytest and mypy both fail. Either the documentation needs to say that pip3 install -r requirements_test_all.txt needs to be run explicitly, or script/setup needs to run it for the user.