Tox should try and test the packaged code rather than the checked out code

[Bachmann1234]

Its nice when testing to test the code in CI for the tests to reflect the production environment as closely as possible. @nicoddemus pointed out that the current config is only testing the checked out code and linked to this article to explain the limitation https://hynek.me/articles/testing-packaging/

https://github.com/Bachmann1234/diff-cover/pull/106#discussion_r287806181

Would be nice to update the tox config to test the packaged code.