Fix tests with updated constraints

[bmw]

When updating constraints.txt, importlib-metadata and mypy currently need to be pinned back for tests to continue to pass. I think this should ideally be fixed.

Also, importlib-metadata is only required by flake8 in Python < 3.8 so if you install josepy with pip using Python 3.8+, this package (and its dependency zipp) won't be included in the pip freeze output. I personally think it may be worth switching this repo to use something like poetry which better handles these cases. I think we could have a more traditional poetry setup here than what we current do in the Certbot repo. This is a larger project though.