Enable linter, update for python3

Add a new job to run "tox -e linters" to doc8 the documents.

Remove the black listing of D000, this is not needed for this repo.
Update tox.ini to use Python3 everywhere.

Change-Id: I5ea55e0000efad1dc4c912fd61baaefb8af09973

.zuul.yaml

@@ -2,3 +2,9 @@
     templates:
       - openstack-manuals-build-translation
       - openstack-manuals-jobs
+    check:
+      jobs:
+        - openstack-tox-linters
+    gate:
+      jobs:
+        - openstack-tox-linters

tox.ini

@@ -4,7 +4,7 @@ envlist = linters,publishdocs
 skipsdist = True
 
 [testenv]
-basepython = python2
+basepython = python3
 setenv =
    VIRTUAL_ENV={envdir}
 deps = -r{toxinidir}/test-requirements.txt
@@ -16,18 +16,15 @@ whitelist_externals =
   sed
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [testenv:linters]
-basepython = python3
 commands =
      doc8 -e '' security-notes
      doc8 -e '' security-guide
      doc8 -e '' security-threat-analysis
 
 [testenv:publishdocs]
-basepython = python3
 # Prepare all documents so that they can get published on
 # docs.openstack.org with just copying publish-docs/* over.
 commands =
@@ -35,7 +32,6 @@ commands =
   {toxinidir}/tools/build-all-rst.sh
 
 [testenv:buildlang]
-basepython = python3
 # Run as "tox -e buildlang -- $LANG"
 whitelist_externals =
     doc-tools-check-languages
@@ -44,18 +40,15 @@ commands =
     doc-tools-check-languages doc-tools-check-languages.conf test {posargs}
 
 [testenv:publishlang]
-basepython = python3
 whitelist_externals = doc-tools-check-languages
 commands = doc-tools-check-languages doc-tools-check-languages.conf test all
 
 [testenv:docs]
-basepython = python3
 commands =
   # Build and copy RST Guides
   {toxinidir}/tools/build-all-rst.sh
 
 [testenv:generatepot-rst]
-basepython = python3
 # Generate POT files for translation, needs {posargs} like:
 # tox -e generatepot-rst -- security-guide
 commands = {toxinidir}/tools/generatepot-rst.sh {posargs}
@@ -66,6 +59,3 @@ commands = {toxinidir}/tools/generatepot-rst.sh {posargs}
 ignore-path = security-notes/OSSN-0047,security-notes/OSSN-0068,common,security-guide/build,security-threat-analysis/build
 # File extensions to use
 extensions = .rst,.txt
-# Disable some doc8 checks:
-# D000: Check RST validity (cannot handle lineos directive)
-ignore = D000