travis: update sphinx release

Try to avoid the following error:

 File "/home/travis/virtualenv/python3.9.1/lib/python3.9/site-packages/docutils/writers/html5_polyglot/__init__.py", line 445, in section_title_tags

    if (ids and self.settings.section_self_link

AttributeError: 'Values' object has no attribute 'section_self_link'

.travis.yml

@@ -12,7 +12,7 @@ env:
 
 install:
     - sudo apt-get install sshpass
-    - pip install -r ${BUILD_DIR}/nscom/requirements.txt -r ${BUILD_DIR}/nsent/requirements.txt sphinx==3.2.1
+    - pip install -r ${BUILD_DIR}/nscom/requirements.txt -r ${BUILD_DIR}/nsent/requirements.txt sphinx==3.4.3
 
 script:
     - pushd ${BUILD_DIR}/