Use RTD config file version 2. (#278)

.readthedocs.yml

@@ -1,8 +1,28 @@
 ---
+# .readthedocs.yaml
+# Read the Docs configuration file
+# See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
+
+# Required
+version: 2
+
+# Set the version of Python and other tools you might need
 build:
-    image: latest
+    os: ubuntu-20.04
+    tools:
+        python: '3.7'
+
+# Build documentation in the docs/ directory with Sphinx
+sphinx:
+    configuration: docs/source/conf.py
+
+# If using Sphinx, optionally build your docs in additional formats such as PDF
+# formats:
+#    - pdf
+
 python:
-    version: 3.7
-    pip_install: true
-    extra_requirements:
-        - docs
+    install:
+        - method: pip
+          path: .
+          extra_requirements:
+              - docs