Readthedocs uses Python 3.8

.readthedocs.yml

@@ -0,0 +1,22 @@
+version: 2
+
+# Build documentation in the docs/ directory with Sphinx
+sphinx:
+  configuration: docs/conf.py
+
+# Build documentation with MkDocs
+#mkdocs:
+#  configuration: mkdocs.yml
+
+# 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: 3.8
+  install:
+    - method: pip
+      path: .
+      extra_requirements:
+        - docs

setup.py

@@ -25,6 +25,7 @@ setup(
         "unidecode",
     ],
     extras_require={
+        "docs": ["sphinx", "sphinx_rtd_theme"],
         "go": ["sphinxcontrib-golangdomain"],
         "dotnet": ["sphinxcontrib-dotnetdomain"],
     },

tox.ini

@@ -42,9 +42,8 @@ commands =
     pylint {posargs:autoapi}
 
 [testenv:docs]
-deps =
-    Sphinx~=3.2.0
-    sphinx_rtd_theme
+extras =
+    docs
 changedir = {toxinidir}/docs
 commands =
     sphinx-build -b html -d {envtmpdir}/doctrees . {envtmpdir}/html