Fix ReadTheDocs CI (#1827)

* Copied .readthedocs.yaml from Celery repo

* Updated version_dev and version_stable in docs/conf.py

.readthedocs.yaml

@@ -0,0 +1,26 @@
+# 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:
+  os: ubuntu-20.04
+  tools:
+    python: "3.9"
+
+# Build documentation in the docs/ directory with Sphinx
+sphinx:
+   configuration: docs/conf.py
+
+# If using Sphinx, optionally build your docs in additional formats such as PDF
+# formats:
+#    - pdf
+
+# Optionally declare the Python requirements required to build your docs
+python:
+   install:
+    - method: pip
+      path: .
+    - requirements: requirements/docs.txt

docs/conf.py

@@ -5,8 +5,8 @@ from sphinx_celery import conf
 globals().update(conf.build_config(
     'kombu', __file__,
     project='Kombu',
-    version_dev='5.1',
-    version_stable='5.0',
+    version_dev='6.0',
+    version_stable='5.3',
     canonical_url='https://kombu.readthedocs.io/',
     webdomain='kombu.readthedocs.io',
     github_project='celery/kombu',