Fix doc build (#3126)

* Fix doc build

ReadTheDocs build has been broken for a while due to incompatibilities between
commonmark, recommonmark, and sphinx. See:
* "Recommonmark not working with Sphinx 1.6"
  https://github.com/rtfd/recommonmark/issues/73
* "CommonMark 0.6.0 breaks compatibility"
  https://github.com/rtfd/recommonmark/issues/24
For now, we fix the versions to get the build working again

* Fix search bar

doc/conf.py

@@ -14,7 +14,6 @@
 import sys
 import os, subprocess
 import shlex
-import urllib
 # If extensions (or modules to document with autodoc) are in another directory,
 # add these directories to sys.path here. If the directory is relative to the
 # documentation root, use os.path.abspath to make it absolute, like shown here.
@@ -164,8 +163,14 @@ def setup(app):
     # Add hook for building doxygen xml when needed
     # no c++ API for now
     # app.connect("builder-inited", generate_doxygen_xml)
-    urllib.urlretrieve('https://code.jquery.com/jquery-2.2.4.min.js',
-                       '_static/jquery.js')
+
+    # urlretrieve got moved in Python 3.x
+    try:
+      from urllib import urlretrieve
+    except ImportError:
+      from urllib.request import urlretrieve
+    urlretrieve('https://code.jquery.com/jquery-2.2.4.min.js',
+                '_static/jquery.js')
     app.add_config_value('recommonmark_config', {
             'url_resolver': lambda url: github_doc_root + url,
             'enable_eval_rst': True,