install mongoengine for readthedocs build to work

.readthedocs.yml

@@ -9,10 +9,10 @@ version: 2
 sphinx:
   configuration: docs/conf.py
 
-# Optionally build your docs in additional formats such as PDF
-formats:
-  - pdf
-
 # Optionally set the version of Python and requirements required to build your docs
 python:
   version: 3.7
+  install:
+    - requirements: docs/requirements.txt
+    - method: setuptools
+      path: mongoengine