Switch to RTD v2 config

.readthedocs.yml

+# Read the Docs configuration file
+# https://docs.readthedocs.io/en/stable/config-file/v2.html
+
+version: 2
+
 python:
-  version: 3
-  extra_requirements:
-  - docs
-  pip_install: false
-  requirements: docs/requirements.txt
+  install:
+  - requirements: docs/requirements.txt