Trying to fix readthedocs build.

.readthedocs.yml

@@ -0,0 +1,3 @@
+python:
+  install:
+    - requirements: doc/requirements.txt

doc/conf.py

@@ -11,11 +11,18 @@
 # All configuration values have a default; values that are commented out
 # serve to show the default.
 
-from __future__ import unicode_literals
-
+from unittest.mock import Mock as MagicMock
 import os
+import sys
 
-# import sys
+
+class Mock(MagicMock):
+    @classmethod
+    def __getattr__(cls, name):
+            return MagicMock()
+
+MOCK_MODULES = ['rrdtool']
+sys.modules.update((mod_name, Mock()) for mod_name in MOCK_MODULES)
 
 # 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