BUG: Temporarily pin the version of Sphinx to 3.3.1 in requirements.
A recent change in Sphinx v3.4.0 is breaking pandas' CI *Build documentation* step.

Until this is resolved, we can temporarily pin Sphinx to the previous version (3.3.1).

_Submitted in response to @jreback in https://github.com/pandas-dev/pandas/issues/38593#issuecomment-748690637_

------

Here's some info that might help for getting this sorted, long term:

Sphinx Changelog: Release 3.4.0 (released Dec 20, 2020)
https://www.sphinx-doc.org/en/master/changes.html#release-3-4-0-released-dec-20-2020

CI build logs from some passing and failing builds:

* A successful build using Sphinx 3.3.1 - https://pastebin.com/raw/jLwVAJsP
* A failing build using Sphinx 3.4.0 - https://pastebin.com/raw/bpHTm2DD
* A here's a diff between these two logs - https://pastebin.com/psJ5hqP8

While passing and failing builds both emit warnings, the following warnings are exclusive to the failing builds under Sphinx 3.4.0:
```
/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
```
lgtm ping in green thanks @shawnbrown 