This is a first step towards implementing the plan I described at https://github.com/certbot/certbot/issues/7909#issuecomment-1448675456 which got a +1 from both Erica and Will. Similar changes for our other packages will be made in followup PRs to try and make this easier to review.
It may be helpful to look at https://github.com/certbot/certbot/pull/7600 when reviewing this PR where we did something similar in the past.
The value of `ignore-paths` in `.pylintrc` should work on Windows based on https://pylint.readthedocs.io/en/latest/user_guide/configuration/all-options.html#ignore-paths and the fact that on macOS/linux, changing path delimiters to `\` still causes these directories to be ignored.
I started testing this for mypy as well, but mypy doesn't current pass for us on Windows so I didn't bother and took this opportunity to remove it from the default environments in `tox.ini`. I'll update https://github.com/certbot/certbot/issues/7803 to mention that the value of `exclude` in `mypy.ini` may need to be tweaked if anyone works on that issue.
* make acme tests internal
* no mypy-win
* Move acme tests to tests/ directory outside of acme module
* Fix call to messages_test in client_test
* Move test_util.py and testdata/ into tests/
* Update manifest to package tests
* Exclude pycache and .py[cod]
