CLN: Change assert_([not] isinstance(a,b)) to specialized forms
Work on #6175. Changes instances of assert_([not] isinstance(a,b)) to specialized assert[Not]IsInstance(a, b).

I will work on the Travis problems. (These did not get picked up locally, as I am running with 2.7.)
 rebase on master
these r already fixed 
 I fetched latest master and rebased (also, changed one superclass, to use tm.TestCase instead of unittest.TestCase). The two errors I am still seeing on Travis are "ImportError: No module named pandas".
 Travis is being flaky today...
 @bwignall rebase on master and travis should be ok now.....setuptools had changed and caused the install to fail
 @bwignall thanks!
