Implement doc8 checks

Change-Id: Ic830833f6abef29a5fc5ca13500a9a8d9a7c036d

requirements.txt

@@ -5,3 +5,4 @@
 # this is required for the docs build jobs
 sphinx!=1.3b1,<1.3,>=1.2.1 # BSD
 oslosphinx>=4.7.0 # Apache-2.0
+doc8 # Apache-2.0

tox.ini

@@ -23,16 +23,23 @@ passenv =
 whitelist_externals =
     bash
 setenv =
+    PYTHONUNBUFFERED=1
     VIRTUAL_ENV={envdir}
 
 
+[testenv:docs]
+commands=
+    bash -c "rm -rf doc/build"
+    doc8 doc
+    python setup.py build_sphinx
+
+
+[doc8]
+# Settings for doc8:
+extensions = .rst
+
+
 # environment used by the -infra templated docs job
 [testenv:venv]
 commands =
     {posargs}
-
-
-[testenv:docs]
-commands=
-    bash -c "rm -rf doc/build"
-    python setup.py build_sphinx