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

Work on #6175. Changes instances of assert_(a is [not] b) to specialized assertIs[Not](a, b).
Merge pull request #6392 from bwignall/assert_6175_isisnot

CLN: Change assert_(a is [not] b) to specialized forms
BUG: Regression in chained getitem indexing with embedded list-like from 0.12 (6394)