[3.7] Doc: Bump sphinx. (GH-10676) (GH-10803) (#16522)

(cherry picked from commit 2db96ae)

Co-authored-by: Julien Palard <julien@palard.fr>
(cherry picked from commit 23a98ab)

Author: vstinner

.travis.yml

@@ -40,7 +40,7 @@ matrix:
         - cd Doc
         # Sphinx is pinned so that new versions that introduce new warnings won't suddenly cause build failures.
         # (Updating the version is fine as long as no warnings are raised by doing so.)
-        - python -m pip install sphinx~=1.6.1 blurb
+        - python -m pip install sphinx==1.8.2 blurb
       script:
         - make check suspicious html SPHINXOPTS="-q -W -j4"
     - os: linux