Fix warning in config; sphinx head works with all 3.x

Fix warning - travis ci expects dist: to be a value not a list.

Newest sphinx work on 3.6 so simplify sphynx install for 3.x.

Author: rouilj

.travis.yml

@@ -16,8 +16,7 @@ branches:
 #    - default
 #    - maint-1.6
 
-dist:
-  - focal
+dist: focal
 
 #  - pypy3
 python:
@@ -55,8 +54,7 @@ before_install:
   # Sphinx required to build the xapian python bindings. Use 1.8.5 on
   # older python and newest on newer.
   - if [[ $TRAVIS_PYTHON_VERSION == "2."* ]]; then pip install sphinx==1.8.5; fi
-  - if [[ $TRAVIS_PYTHON_VERSION == "3.[67]" ]]; then pip install sphinx==1.8.5; fi
-  - if [[ $TRAVIS_PYTHON_VERSION != "3.[67]" ]] && [[ $TRAVIS_PYTHON_VERSION == '3.'* ]] ; then pip install sphinx; fi
+  - if [[ $TRAVIS_PYTHON_VERSION == '3.'* ]] ; then pip install sphinx; fi
   - if [[ $TRAVIS_PYTHON_VERSION == "nightly" ]]; then pip install sphinx; fi
   - XAPIAN_VER=$(dpkg -l libxapian-dev | tail -n 1 | awk '{print $3}' | cut -d '-' -f 1)
   - cd /tmp