CLN/TST: #6175, specializing assert_ (WIP)
Helping with the specialization request. Work-in-progress. Updated tests pass locally.

Looks good. I'd merge on green but will wait until 0.13.1 is released, out of caution.
Please have a look at guidelines on commit subjects in [https://github.com/pydata/pandas/blob/master/CONTRIBUTING.md](CONTRIBUTING.md). If you're comfortable with git,
you may amend, if not leave them as is and we'll take care of it.

If you wish to do more, I suggest you open a seperate PR with more work. 
Keeping PRs small makes it easier for us to review and merge with confidence.

Thanks for contributing.
 Thanks for your comment, y-p. My apologies on the mistake; I am still learning my way around git. I will open another PR with more on this topic shortly, hopefully that goes more smoothly.
 On the contrary, if that's the case you've done very well.
 https://github.com/pydata/pandas/pull/6198 superceded this.

@bwignall, it's good practice to always start a new PR by branching against the current
git master. If you include the commits from one PR in another (sometimes one PR depends
on another) please explicitly state that you've done so and why, or close the superceded
one if it has been.

Git takes some getting used to, don't beat youself up. Thanks again for contributing.
 @y-p,

My apologies (again) for the elementary mistakes. I appreciate your patience and your suggestions.

I am happy to help. I will endeavor to do so more efficiently in the future.
