Fix definition of `cov` fixture for pytest 3
Fix definition of `cov` fixture for pytest 3

With pytest 3.0 (not released yet) the following warning gets emitted:

> WC1 None pytest_funcarg__cov: declaring fixtures using
> "pytest_funcarg__" prefix is deprecated and scheduled to be removed in
> pytest 4.0.  Please remove the prefix and use the @pytest.fixture decorator
> instead.

pytest-dev/pytest-cov,schlamar/pytest-cov