Example:
https://travis-ci.org/python/cpython/jobs/590339147

$ python --version

Python 3.6.3

$ pip --version

pip 9.0.1 from /home/travis/virtualenv/python3.6.3/lib/python3.6/site-packages (python 3.6)

Could not locate requirements.txt. Override the install: key in your .travis.yml to install dependencies.
before_script.1

0.00s$ cd Doc
before_script.2

8.58s$ python -m pip install sphinx~=1.6.1 blurb

1.88s$ make check suspicious html SPHINXOPTS="-q -W -j4"

python3 tools/rstlint.py -i tools -i ./venv -i README.rst

No problems found.

make[1]: Entering directory `/home/travis/build/python/cpython/Doc'

mkdir -p build

Building NEWS from Misc/NEWS.d with blurb

PATH=./venv/bin:$PATH sphinx-build -b suspicious -d build/doctrees -D latex_elements.papersize= -q -W -j4 . build/suspicious 

Sphinx version error:

This project needs at least Sphinx v1.8 and therefore cannot be built with this version.

make[1]: *** [build] Error 1

make[1]: Leaving directory `/home/travis/build/python/cpython/Doc'

Suspicious check complete; look for any errors in the above output or in build/suspicious/suspicious.csv.  If all issues are false positives, append that file to tools/susp-ignored.csv.

make: *** [suspicious] Error 1

The command "make check suspicious html SPHINXOPTS="-q -W -j4"" exited with 2.
cache.2

store build cache

This issue prevents to merge the following 3.5 pull requests:
https://github.com/python/cpython/pulls?utf8=%E2%9C%93&q=is%3Apr+is%3Aopen+base%3A3.5+

I suggest to backport https://github.com/python/cpython/pull/10676 to 3.5: use Sphinx 1.8.2 in .travis.yml. Commit in the 3.6 branch:

commit 23a98abd4256f931ed89b65ec6babd4f06dbff97
Author: Miss Islington (bot) <31488909+miss-islington@users.noreply.github.com>
Date:   Sat Dec 8 21:34:30 2018 -0800

    [3.7] Doc: Bump sphinx. (GH-10676) (GH-10803)
    
    (cherry picked from commit 2db96ae7444880d66d4ef65abab8a5e6ff328711)
    
    Co-authored-by: Julien Palard <julien@palard.fr>

diff --git a/.travis.yml b/.travis.yml
index ed4e46c782..8250fd91b3 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -43,7 +43,7 @@ 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

I proposed PR 16522 to update Sphinx from version 1.6.1 to 1.8.2.

Larry merged my fix:

commit edd9bc9cc99c919bdc87e5955ec8ad93c295fbe2
Author: Victor Stinner <vstinner@redhat.com>
Date:   Tue Oct 8 19:12:21 2019 +0200

    [3.7] Doc: Bump sphinx. (GH-10676) (GH-10803) (#16522)
    
    (cherry picked from commit 2db96ae7444880d66d4ef65abab8a5e6ff328711)
    
    Co-authored-by: Julien Palard <julien@palard.fr>
    (cherry picked from commit 23a98abd4256f931ed89b65ec6babd4f06dbff97)


The doc of job Travis CI is back to green ;-)