fix(tests): add missing dependency on doc8 for "tox -e doc8"

--HG--
branch : 3.27

tox.ini

@@ -43,6 +43,8 @@ commands =
 
 [testenv:doc8]
 skip_install = true
+deps =
+  doc8
 commands=
     doc8 --ignore-path doc/_build --ignore-path .tox --ignore-path .hg doc