ENH: Allow absolute precision in assert_almost_equal (#13357)
closes #9457

Greetings! This is my first pull-request on an open source project, so I hope I did not miss anything at least too obvious... Thank you, in advance :smile: 

I do have a few questions:

* Should I add the `versionadded` directive in all docstrings that were changed?
* Which "whatsnew" release notes should I add this change to? v1.0.0?
* Should #9457 also be closed, as from my understanding this was also the underlying issue?
* Should I be the one ticking the checkboxes below?

This fixes the issue where:

    # Fails because it's doing (1 - .1 / .1001)
    assert_almost_equal(0.1, 0.1001, check_less_precise=True)

    # Works as intuitively expected
    assert_almost_equal(
        0.1, 0.1001,
        atol=0.01,
    )



Commit message below:
-------

This commit makes `assert_almost_equal` accept both relative and
absolute precision when comparing numbers, through two new keyword
arguments: `rtol`, and `atol`, respectively.

Under the hood, `_libs.testing.assert_almost_equal` is now calling
`math.isclose`, instead of an adaptaion of
[numpy.testing.assert_almost_equal](https://docs.scipy.org/doc/numpy-1.17.0/reference/generated/numpy.testing.assert_almost_equal.html).

- [x] closes #13357 
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry