BUG: Temporarily pin the version of Sphinx to 3.3.1 in requirements. (pandas-dev#38598)

* BUG: Temporarily pin the version of Sphinx to 3.3.1 in requirements.

Currently, the CI "Build documentation" step is failing with Sphinx 3.4.0.

* Temporarily pin Sphinx version in environment.yml (See 1a03918).

* Correct version definition in environment.yml file.

Author: shawnbrown

environment.yml

@@ -31,7 +31,7 @@ dependencies:
   # documentation
   - gitpython  # obtain contributors from git for whatsnew
   - gitdb
-  - sphinx
+  - sphinx=3.3.1
 
   # documentation (jupyter notebooks)
   - nbconvert>=5.4.1

requirements-dev.txt

@@ -17,7 +17,7 @@ pycodestyle
 pyupgrade
 gitpython
 gitdb
-sphinx
+sphinx==3.3.1
 nbconvert>=5.4.1
 nbsphinx
 pandoc