have Travis use pip3 for Sphinx 2.x

Author: sommersoft

.travis.yml

@@ -83,7 +83,7 @@ before_script:
   - pip3 install --user sh click
   - ([[ -z "$TRAVIS_TESTS" ]] || sudo pip install --upgrade cpp-coveralls)
   - (! var_search "${TRAVIS_TESTS-}" docs || sudo apt-get install -y librsvg2-bin)
-  - (! var_search "${TRAVIS_TESTS-}" docs || pip install --user Sphinx sphinx-rtd-theme recommonmark sphinxcontrib-svg2pdfconverter)
+  - (! var_search "${TRAVIS_TESTS-}" docs || pip3 install --user Sphinx sphinx-rtd-theme recommonmark sphinxcontrib-svg2pdfconverter)
   - (! var_search "${TRAVIS_TESTS-}" translations || pip3 install --user polib)
 
   # report some good version numbers to the build