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.
    
    The convenience wrapper for running the CLI direct from the
    top-level directory and the `checkwiki` test script are updated
    to still work with the module moved like this. We also update the
    tox (CI) configuration to work with it, 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>