tox: Keeping going with docs

Sphinx 1.8 introduced [1] the '--keep-going' argument which, as its name
suggests, keeps the build running when it encounters non-fatal errors.
This is exceptionally useful in avoiding a continuous edit-build loop
when undertaking large doc reworks where multiple errors may be
introduced.

[1] https://github.com/sphinx-doc/sphinx/commit/e3483e9b045

Change-Id: Ie373018aafc05f7ea859a73c55163840dee60b56

doc/requirements.txt

@@ -3,7 +3,7 @@
 # process, which may cause wedges in the gate later.
 
 openstackdocstheme>=1.30.0 # Apache-2.0
-sphinx>=1.6.2,!=1.6.6,!=1.6.7,<2.0.0;python_version=='2.7'  # BSD
-sphinx>=1.6.2,!=1.6.6,!=1.6.7,!=2.1.0;python_version>='3.4' # BSD
+sphinx>=1.8.0,<2.0.0;python_version=='2.7' # BSD
+sphinx>=1.8.0,!=2.1.0;python_version>='3.4' # BSD
 sphinxcontrib-svg2pdfconverter>=0.1.0 # BSD
 reno>=2.5.0 # Apache-2.0

tox.ini

@@ -49,7 +49,7 @@ commands = {posargs}
 [testenv:docs]
 basepython = python3
 deps = -r{toxinidir}/doc/requirements.txt
-commands = sphinx-build -W -b html doc/source doc/build/html
+commands = sphinx-build -W --keep-going -b html doc/source doc/build/html
 
 [testenv:pdf-docs]
 basepython = python3
@@ -58,7 +58,7 @@ deps = {[testenv:docs]deps}
 whitelist_externals =
   make
 commands =
-  sphinx-build -W -b latex doc/source doc/build/pdf
+  sphinx-build -W --keep-going -b latex doc/source doc/build/pdf
   make -C doc/build/pdf
 
 [flake8]
@@ -77,7 +77,8 @@ basepython = python3
 deps =
   -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/master/upper-constraints.txt}
   -r{toxinidir}/doc/requirements.txt
-commands = sphinx-build -a -E -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
+commands =
+  sphinx-build -a -E -W -d releasenotes/build/doctrees --keep-going -b html releasenotes/source releasenotes/build/html
 
 [testenv:lower-constraints]
 basepython = python3