DOC: Add datatest package to list of third-party extension accessors.
This change adds the `datatest` package to the list of third-party extension accessors. Since `datatest` supports more than `pandas`, I wasn't sure if it was appropriate to add it to the **Data cleaning and validation** section, so I only added it to the accessors list for now.

Unfortunately, the "Classes" column was not wide enough--so this patch affects all rows in the table. This shouldn't happen again as the column is now wide enough to accommodate all of the classes that support extension accessors.
I'm not sure what to make of this. The changes I made don't seem to be an issue according to the "Build documentation" logs:

* A previous, successful build - https://pastebin.com/raw/jLwVAJsP
* This current, failing build - https://pastebin.com/raw/bpHTm2DD
* A here's a diff between these two logs - https://pastebin.com/psJ5hqP8

The successful build used Sphinx v3.3.1 and the failing build used Sphinx v3.4.0. In the successful build, there are warnings emitted *after* the success. But in the failing build, these same warnings are emitted during the process--causing it to fail.

EDIT: Correction, there are additional warnings in the failing build that aren't in the passing build (see following comment). This seems to be happening for files totally unrelated to the change. The failing **Build documentation** step emits warnings that the passing build did not. This seems to have started with the newly published Sphinx v3.4.0 (released earlier today). The new warnings are as follows:

```
/home/runner/work/pandas/pandas/doc/source/docstring of pandas._libs.tslibs.offsets.BusinessDay.rst:18: WARNING: autosummary: failed to import base
/home/runner/work/pandas/pandas/doc/source/docstring of pandas._libs.tslibs.offsets.BusinessDay.rst:18: WARNING: autosummary: failed to import offset
/home/runner/work/pandas/pandas/doc/source/docstring of pandas._libs.tslibs.offsets.BusinessDay.rst:40: WARNING: autosummary: failed to import __call__
/home/runner/work/pandas/pandas/doc/source/docstring of pandas._libs.tslibs.offsets.BusinessDay.rst:40: WARNING: autosummary: failed to import rollback
/home/runner/work/pandas/pandas/doc/source/docstring of pandas._libs.tslibs.offsets.BusinessDay.rst:40: WARNING: autosummary: failed to import rollforward
/home/runner/work/pandas/pandas/doc/source/docstring of pandas._libs.tslibs.offsets.BusinessMonthBegin.rst:18: WARNING: autosummary: failed to import base
/home/runner/work/pandas/pandas/doc/source/docstring of pandas._libs.tslibs.offsets.BusinessMonthBegin.rst:36: WARNING: autosummary: failed to import __call__
/home/runner/work/pandas/pandas/doc/source/docstring of pandas._libs.tslibs.offsets.BusinessMonthBegin.rst:36: WARNING: autosummary: failed to import rollback
/home/runner/work/pandas/pandas/doc/source/docstring of pandas._libs.tslibs.offsets.BusinessMonthBegin.rst:36: WARNING: autosummary: failed to import rollforward
/home/runner/work/pandas/pandas/doc/source/docstring of pandas._libs.tslibs.offsets.BusinessMonthEnd.rst:18: WARNING: autosummary: failed to import base
/home/runner/work/pandas/pandas/doc/source/docstring of pandas._libs.tslibs.offsets.BusinessMonthEnd.rst:36: WARNING: autosummary: failed to import __call__
/home/runner/work/pandas/pandas/doc/source/docstring of pandas._libs.tslibs.offsets.BusinessMonthEnd.rst:36: WARNING: autosummary: failed to import rollback
/home/runner/work/pandas/pandas/doc/source/docstring of pandas._libs.tslibs.offsets.BusinessMonthEnd.rst:36: WARNING: autosummary: failed to import rollforward
/home/runner/work/pandas/pandas/doc/source/docstring of pandas._libs.tslibs.offsets.CustomBusinessMonthBegin.rst:18: WARNING: autosummary: failed to import base
/home/runner/work/pandas/pandas/doc/source/docstring of pandas._libs.tslibs.offsets.CustomBusinessMonthBegin.rst:18: WARNING: autosummary: failed to import cbday_roll
/home/runner/work/pandas/pandas/doc/source/docstring of pandas._libs.tslibs.offsets.CustomBusinessMonthBegin.rst:18: WARNING: autosummary: failed to import month_roll
/home/runner/work/pandas/pandas/doc/source/docstring of pandas._libs.tslibs.offsets.CustomBusinessMonthBegin.rst:18: WARNING: autosummary: failed to import offset
/home/runner/work/pandas/pandas/doc/source/docstring of pandas._libs.tslibs.offsets.CustomBusinessMonthBegin.rst:43: WARNING: autosummary: failed to import __call__
/home/runner/work/pandas/pandas/doc/source/docstring of pandas._libs.tslibs.offsets.CustomBusinessMonthBegin.rst:43: WARNING: autosummary: failed to import rollback
/home/runner/work/pandas/pandas/doc/source/docstring of pandas._libs.tslibs.offsets.CustomBusinessMonthBegin.rst:43: WARNING: autosummary: failed to import rollforward
/home/runner/work/pandas/pandas/doc/source/docstring of pandas._libs.tslibs.offsets.CustomBusinessMonthEnd.rst:18: WARNING: autosummary: failed to import base
/home/runner/work/pandas/pandas/doc/source/docstring of pandas._libs.tslibs.offsets.CustomBusinessMonthEnd.rst:18: WARNING: autosummary: failed to import cbday_roll
/home/runner/work/pandas/pandas/doc/source/docstring of pandas._libs.tslibs.offsets.CustomBusinessMonthEnd.rst:18: WARNING: autosummary: failed to import month_roll
/home/runner/work/pandas/pandas/doc/source/docstring of pandas._libs.tslibs.offsets.CustomBusinessMonthEnd.rst:18: WARNING: autosummary: failed to import offset
/home/runner/work/pandas/pandas/doc/source/docstring of pandas._libs.tslibs.offsets.CustomBusinessMonthEnd.rst:43: WARNING: autosummary: failed to import __call__
/home/runner/work/pandas/pandas/doc/source/docstring of pandas._libs.tslibs.offsets.CustomBusinessMonthEnd.rst:43: WARNING: autosummary: failed to import rollback
/home/runner/work/pandas/pandas/doc/source/docstring of pandas._libs.tslibs.offsets.CustomBusinessMonthEnd.rst:43: WARNING: autosummary: failed to import rollforward
/home/runner/work/pandas/pandas/doc/source/docstring of pandas._libs.tslibs.offsets.CustomBusinessDay.rst:18: WARNING: autosummary: failed to import base
/home/runner/work/pandas/pandas/doc/source/docstring of pandas._libs.tslibs.offsets.CustomBusinessDay.rst:18: WARNING: autosummary: failed to import offset
/home/runner/work/pandas/pandas/doc/source/docstring of pandas._libs.tslibs.offsets.CustomBusinessDay.rst:40: WARNING: autosummary: failed to import __call__
/home/runner/work/pandas/pandas/doc/source/docstring of pandas._libs.tslibs.offsets.CustomBusinessDay.rst:40: WARNING: autosummary: failed to import rollback
/home/runner/work/pandas/pandas/doc/source/docstring of pandas._libs.tslibs.offsets.CustomBusinessDay.rst:40: WARNING: autosummary: failed to import rollforward
``` Here's the changelog for Sphinx 3.4.0:

https://www.sphinx-doc.org/en/master/changes.html#release-3-4-0-released-dec-20-2020
 e should pin sphinx to the earlier version
for now (and open an issue to fix)

want to do a PR? > want to do a PR?

Sure. I'll post in a minute. thanks @shawnbrown 