Move module under src/, add pyproject.toml, drop pytest-cov
    
    This moves the Python module under src/ , for reasons discussed
    at https://hynek.me/articles/testing-packaging/ : essentially
    it's good practice to *not* have the module directly importable
    from the top-level working directory.
    
    We update the tox (CI) configuration to work with the change, by
    building an sdist and testing that, rather than testing the
    contents of the working directory: this is a stronger test (as we
    ultimately deploy via sdist, so we need to make sure the sdist we
    build works). We also update setup.py for the change.
    
    We add pyproject.toml to express that setuptools-git is needed
    to build the sdist correctly, and we switch to running coverage
    and having it call pytest versus using pytest-cov as this is
    necessary for coverage diffing to work correctly with the layout
    change.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>