Move Sphinx docs build from Python 3.5 to 3.7 since 3.7 is what ReadTheDocs now uses for builds

Author: tleonhardt

.travis.yml

@@ -21,7 +21,7 @@ matrix:
           dist: xenial
           env: TOXENV=py39
         - os: linux
-          python: 3.5
+          python: 3.7
           env: TOXENV=docs
 #        # Warning: Don't try to use code coverage analysis with pypy as it is insanely slow
 #        - os: linux

tox.ini

@@ -13,7 +13,7 @@ commands =
   codecov
 
 [testenv:docs]
-basepython = python3.5
+basepython = python3.7
 deps =
   sphinx
   sphinx-rtd-theme