Adjust readthedocs to not use requirements files

Remove .yml since RTD considers it deprecated.

.gitignore

@@ -6,7 +6,7 @@
 !.gitattributes
 !.gitignore
 !.pre-commit-config.yaml
-!.readthedocs.yml
+!.readthedocs.yaml
 !.github/
 !.docker/
 

.readthedocs.yaml

@@ -0,0 +1,22 @@
+# 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:
+    - method: pip
+      path: .
+      extra_requirements:
+        - docs

.readthedocs.yml

@@ -1,10 +0,0 @@
-build:
-  image: latest
-
-python:
-  version: 3.6
-
-formats:
-  - pdf
-
-requirements_file: requirements/main.txt