ENH: add statsmodels wheel

Also removes the statsmodels/pandas faking part of ci/install.sh by using a
statsmodels wheel
CLN: proper version checking for statsmodels testWLS
Merge pull request #4981 from cpcloud/statsmodels-ci-addition

CI/ENH: test against an older version of statsmodels
API: properly box numeric timedelta ops on Series (GH4984)