Fix readthedocs build that failed by making it use python3 instead of default python2.7

.readthedocs.yml

@@ -0,0 +1,20 @@
+# .readthedocs.yml
+# Read the Docs configuration file
+# See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
+
+# Required
+version: 2
+
+# Build documentation in the docs/ directory with Sphinx
+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

requirements.txt

@@ -1,3 +1,3 @@
-pymongo>=3.4
+pymongo>=3.11
-Sphinx==1.5.5
+Sphinx==3.2.1
-sphinx-rtd-theme==0.2.4
+sphinx-rtd-theme==0.5.0