dev-requirements: pytest-cov==2.6.1 for '--no-cov'

Bump the version of pytest-cov, so that we pick up the fix for
the '--no-cov' option breaking in pytest 4.0:

    https://github.com/pytest-dev/pytest-cov/pull/230

This was the error you would get prior to this version:

    _pytest.warning_types.RemovedInPytest4Warning: config.warn has been
    deprecated, use warnings.warn instead

Although it says 'deprecated', it did result in
termination with stack trace.