add RTD config file to fix Python version issue (#1453)

Signed-off-by: Adrian Sutton <adrian.sutton@consensys.net>

Jenkinsfile

@@ -161,7 +161,9 @@ try {
         node {
             checkout scm
             stage(stage_name + 'Build') {
-                def container = docker.image("python:3-alpine").inside() {
+//                Python image version should be set to the same as in readthedocs.yml
+//                to make sure we test with the same version that RTD will use
+                def container = docker.image("python:3.7-alpine").inside() {
                     try {
                         sh 'pip install -r docs/requirements.txt'
                         sh 'mkdocs build -s'

readthedocs.yml

@@ -0,0 +1,17 @@
+# readthedocs.yml
+
+version: 2
+
+build:
+  image: latest
+
+python:
+#  Python version should be set to this one too in the Jenkinsfile to
+#  make sure we test with the same version that RTD will use
+  version: 3.7
+  install:
+    - requirements: docs/requirements.txt
+
+mkdocs:
+  configuration: mkdocs.yml
+  fail_on_warning: false