Use newer Sphinx also with tox and Read the Docs

Author: Cito

docs/requirements.txt

@@ -1,2 +1,2 @@
-sphinx>=2.2,<3
-sphinx_rtd_theme>=0.4
+sphinx>=3.2,<4
+sphinx_rtd_theme>=0.5,<1

tox.ini

@@ -23,8 +23,8 @@ commands =
 [testenv:docs]
 basepython = python3.8
 deps =
-    sphinx>=2.4,<3
-    sphinx_rtd_theme>=0.4,<1
+    sphinx>=3.2,<4
+    sphinx_rtd_theme>=0.5,<1
 commands =
     sphinx-build -b html -nEW docs docs/_build/html