CLN: Deprecation of assert_
#7131: Run s/self.assert_(/self.assertTrue( on remaining instances I can find. These were the multi-line expressions, and I tried to retain the formatting of the expressions

here's an example of running with `PANDAS_TESTING_MODE='deprecate'` on 3.4: https://travis-ci.org/jreback/pandas/jobs/25480011
 Thanks, @jreback, I will look through that in more detail later today.
