Fix pytest warning about cov

[JukkaL]

Without this I get a pytest warning about cov every time I run it.
The cov plugin still gets loaded and I can still generate coverage
reports, so I assume that the coverage plugin is loaded
automatically. Not sure if we should require a more recent pytest
version.

This is what the warning looked like:

======================= pytest-warning summary ========================
WP1 None Module already imported so can not be re-written: pytest_cov

[gvanrossum]

I see this warning too. I've always just ignored it.

@cpennington Since you added the pytest_cov line to conftest.py, (in #2293) can you explain why you added it, and do you agree that it's fine to remove it? Maybe @gnprice or @ambv know what's going on here?

[cpennington]

I think the pytest_cov plugin is what adds the --cov option used here: https://github.com/python/mypy/blob/master/runtests.py#L106. But we likely can do without that plugin, as well.

[gvanrossum]

OK, confirmed that things still work, so merging.