Consistency between `coverage: codecov` and `coverage: github` for easy transition

[Cadair]

I've just tried out coverage:github on a repo and I found the whole process very frustrating to the point where I gave up.

The main issue is that it seems impossible to use pytest-cov with coverage:github, as I can't see a way to generate the .coverage sqlite data file using pytest-cov.
The upshot of this is that users will presumably need to edit their tox.ini to prefix coverage runs with coverage run. This is too high a burden on users.

The fact that coverage: codecov relies on pytest-cov makes this even more galling, because we know that people who care about coverage reporting are already using pytest-cov so the migration path is very hard.