be explicit in requiring rtd theme for docs

Author: tomato42

.readthedocs.yaml

@@ -7,17 +7,13 @@ version: 2
 
 # Set the version of Python and other tools you might need
 build:
-  os: ubuntu-20.04
+  os: ubuntu-22.04
   tools:
-    python: "3.9"
-    # You can also specify other tool versions:
-    # nodejs: "16"
-    # rust: "1.55"
-    # golang: "1.17"
+    python: "3.11"
 
 # Build documentation in the docs/ directory with Sphinx
 sphinx:
-   configuration: docs/source/conf.py
+  configuration: docs/source/conf.py
 
 # If using Sphinx, optionally build your docs in additional formats such as PDF
 # formats:
@@ -26,4 +22,5 @@ sphinx:
 # Optionally declare the Python requirements required to build your docs
 python:
    install:
+   - requirements: requirements.txt
    - requirements: docs/requirements.txt

docs/requirements.txt

@@ -1 +1 @@
-six
+sphinx-rtd-theme

requirements.txt

@@ -0,0 +1 @@
+six