Mock grpcio when generating docs.

docs/conf.py

@@ -12,6 +12,7 @@
 # serve to show the default.
 
 from email import message_from_string
+import mock
 import os
 from pkg_resources import get_distribution
 import sys
@@ -23,6 +24,20 @@
 ON_READ_THE_DOCS = os.environ.get('READTHEDOCS', None) == 'True'
 LOCAL_READ_THE_DOCS = os.environ.get('LOCAL_RTD', None) == 'True'
 
+
+# See
+# (https://read-the-docs.readthedocs.io/en/latest/faq.html#\
+#  i-get-import-errors-on-libraries-that-depend-on-c-modules)
+
+class Mock(mock.Mock):
+
+    @classmethod
+    def __getattr__(cls, name):
+            return Mock()
+
+
+MOCK_MODULES = ('grpcio')
+
 # 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.

tox.ini

@@ -82,6 +82,7 @@ commands =
     sphinx-build -W -b html -d docs/_build/doctrees docs docs/_build/html
     python {toxinidir}/scripts/verify_included_modules.py
 deps =
+    mock
     Sphinx
     sphinx_rtd_theme
 passenv = {[testenv:system-tests]passenv} SPHINX_RELEASE READTHEDOCS LOCAL_RTD