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).
CLN: Change assert_([not] isinstance(a,b)) to specialized forms

Work on #6175. Change superclass of tests/test_compat, to use tm.TestCase
Merge pull request #6577 from bwignall/assert_6175_isinstance

CLN: Change assert_([not] isinstance(a,b)) to specialized forms