Update RTD config file to v2

.readthedocs.yml

+# .readthedocs.yml
+# Read the Docs configuration file
+# See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
+
+# Required
+version: 2
+
+sphinx:
+  configuration: docs/conf.py
+
+formats: all
+
+python:
+  version: 3.7
+  install:
+    - requirements: requirements-dev.txt

readthedocs.yml

-requirements_file: requirements-dev.txt
-python:
-    version: 3
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