Docs build cleanup

Break doc requirements out to a separate file in the doc/ tree.

Drop upper-bound on sphinx -- the gate job has been pulling from
upper-constraints, anyway.

Clean up a warning:

   WARNING: html_static_path entry '_static' does not exist

Change-Id: I70010029060442fb31d96fce57e8f44a55424b7f

doc/requirements.txt

@@ -0,0 +1 @@
+sphinx>=1.1.2,!=1.2.0,!=1.3b1  # BSD

doc/source/conf.py

@@ -120,7 +120,7 @@ html_theme = 'default'
 # Add any paths that contain custom static files (such as style sheets) here,
 # relative to this directory. They are copied after the builtin static files,
 # so a file named "default.css" will overwrite the builtin "default.css".
-html_static_path = ['_static']
+#html_static_path = ['_static']
 
 # If not '', a 'Last updated on:' timestamp is inserted at every page bottom,
 # using the given strftime format.

test-requirements.txt

@@ -5,4 +5,3 @@
 coverage
 nose
 six>=1.9.0
-sphinx>=1.1.2,!=1.2.0,!=1.3b1,<1.3  # BSD

tox.ini

@@ -26,4 +26,6 @@ commands=
 commands = {posargs}
 
 [testenv:docs]
+deps =
+  -r{toxinidir}/doc/requirements.txt
 commands = python setup.py build_sphinx