diff --git a/test/test_version.py b/test/test_version.py
index ce91f9d..d10e531 100644
--- a/test/test_version.py
+++ b/test/test_version.py
@@ -2,8 +2,8 @@
 
 
 def test_version_fails_gracefully(mocker):
-    target = 'pkg_resources.get_distribution'
-    with mocker.patch(target, side_effect=Exception()):
-        from calibrestekje.__init__ import __version__
+    target = "pkg_resources.get_distribution"
+    mocker.patch(target, side_effect=Exception())
+    from calibrestekje.__init__ import __version__
 
-        assert __version__ == 'unknown'
+    assert __version__ == "unknown"
diff --git a/tox.ini b/tox.ini
index 5ad4b8d..edc1ba8 100644
--- a/tox.ini
+++ b/tox.ini
@@ -34,7 +34,7 @@ commands = isort {posargs:-rc -c} -sp setup.cfg calibrestekje/ test/
 [testenv:format]
 description = format the source
 skipdist = True
-basepython = python3.6
+basepython = python3.8
 deps = black
 commands = black {posargs:--check} calibrestekje/ test/
 
@@ -50,7 +50,9 @@ commands = mypy calibrestekje/ test/
 [testenv:docs]
 description = build the documentation
 skipdist = True
-deps = sphinx
+deps =
+  sphinx
+  sphinx_rtd_theme
 commands = python -m setup build_sphinx
 
 [testenv:docs-livereload]