Coverage: 98.99% (-0.5%) from 99.54% when pulling 756dc68 on eriknw:include_conftest into 0eeebfc on python-graphblas:main.

Okay, so adding conftest.py to MANIFEST.in actually doesn't add it to the wheel--it's only added to the source distribution. It belongs in the source distribution, so this change is still good.

I just added some logic so that slow tests will run (and not emit warnings) when run via pytest --pyargs graphblas. It also runs "randomly". I don't know how to allow custom cli arguments to be passed in when pytest is invoked in this way.