BLD: Move coverage to Python 3.7 branch
BLD: Move coverage to Python 3.7 branch

Move to Python 3.7 env to get leading edge coverage
Move to Python 3.7 env to improve performance when usign openblas

ChadFulton/statsmodels,josef-pkt/statsmodels,ChadFulton/statsmodels,statsmodels/statsmodels,josef-pkt/statsmodels,bashtage/statsmodels,statsmodels/statsmodels,jseabold/statsmodels,josef-pkt/statsmodels,statsmodels/statsmodels,ChadFulton/statsmodels,statsmodels/statsmodels,bashtage/statsmodels,bashtage/statsmodels,bashtage/statsmodels,statsmodels/statsmodels,josef-pkt/statsmodels,ChadFulton/statsmodels,bashtage/statsmodels,josef-pkt/statsmodels,jseabold/statsmodels,jseabold/statsmodels,ChadFulton/statsmodels,josef-pkt/statsmodels,jseabold/statsmodels,ChadFulton/statsmodels,jseabold/statsmodels,statsmodels/statsmodels,bashtage/statsmodels