Upgrade version of doc8

The doc8 module explicitl supports Python 3.6 starting from version
0.8.1

Also remove it from test-requirements as we already have it in tox.ini

Change-Id: Idc66758dc350d9a7a09b4295b08f4423cec288dc

test-requirements.txt

@@ -1,11 +1,10 @@
 # The order of packages is significant, because pip processes them in the order
 # of appearance. Changing the order has an impact on the overall integration
 # process, which may cause wedges in the gate later.
+
 coverage!=4.4,>=4.0 # Apache-2.0
 testtools>=2.2.0 # MIT
 oslotest>=3.2.0 # Apache-2.0
 stestr>=1.0.0 # Apache-2.0
 bandit!=1.6.0,>=1.1.0,<2.0.0 # Apache-2.0
 
-# Doc test requirements
-doc8>=0.6.0 # Apache-2.0

tox.ini

@@ -38,7 +38,7 @@ deps=
     bashate>=0.5.1 # Apache-2.0
     flake8-import-order>=0.17.1 # LGPLv3
     pycodestyle>=2.0.0,<2.7.0 # MIT
-    doc8>=0.6.0 # Apache-2.0
+    doc8>=0.8.1 # Apache-2.0
 whitelist_externals = bash
 commands =
   flake8 {posargs:ironic_python_agent examples}