attempt to fix ReadTheDocs build issue

.readthedocs.yaml

@@ -24,6 +24,6 @@ formats:
   - pdf
 
 # Optionally declare the Python requirements required to build your docs
-#python:
-#  install:
-#    - requirements: docs/requirements.txt
+python:
+  install:
+    - requirements: docs/requirements.txt

docs/requirements.txt

@@ -0,0 +1,2 @@
+sphinx>=4.0
+sphinx_rtd_theme==1.0.0

docs/source/conf.py

@@ -40,7 +40,7 @@ def setup(app):
 
 # If your documentation needs a minimal Sphinx version, state it here.
 #
-needs_sphinx = '4.2.0'
+needs_sphinx = '4.0'
 
 # Add any Sphinx extension module names here, as strings. They can be
 # extensions coming with Sphinx (named 'sphinx.ext.*') or your custom