Set dependencies for readthedocs (#559)

Author: zinwang

.readthedocs.yaml

@@ -5,6 +5,10 @@ build:
   tools:
     python: "3.8"
 
+python:
+  install:
+    - requirements: docs/requirements.txt
+
 sphinx:
    configuration: docs/source/conf.py
 

docs/requirements.txt

@@ -0,0 +1,9 @@
+pillow
+mock==1.0.1
+alabaster>=0.7,<0.8,!=0.7.5
+commonmark==0.9.1
+recommonmark==0.5.0
+sphinx<2
+sphinx-rtd-theme<0.5
+readthedocs-sphinx-ext<2.3
+jinja2<3.1.0