CLN: Changing some 'assert_'s to 'assert[Not]Equal', as per GH6175
CLN: test_plotting, specialize assert_

GH6175: replacing "assert_" with specialized forms of assert
CLN: test_resample, specialize assert_

GH6175: replacing "assert_" with specialized forms of assert. WIP: calls with assert_( (...).all() ) have not been changed.
CLN: test_timedeltas, specialize assert_

GH6175: replacing "assert_" with specialized form assertEqual.
Merge pull request #6198 from  bwignall/assert_6175_2 (reworked)

* bwignall-assert_6175_2:
  CLN: test_timedeltas, specialize assert_
  CLN: test_resample, specialize assert_
  CLN: test_plotting, specialize assert_
  CLN: Changing some 'assert_'s to 'assert[Not]Equal', as per GH6175