update sphinx

note that per the https://docs.readthedocs.io/en/stable/config-file/v2.html#build-tools-python
specify "3" for the latest python version (not 3.x as on github actions)

.readthedocs.yml

@@ -11,7 +11,7 @@ version: 2
 build:
   os: ubuntu-20.04
   tools:
-    python: "3.9"
+    python: "3"
 
 submodules:
     include:
@@ -22,4 +22,4 @@ formats:
 
 python:
     install:
-        - requirements: docs/requirements.txt
+        - requirements: requirements-doc.txt

docs/requirements.txt

@@ -1,8 +0,0 @@
-sphinx<4
-myst-parser==0.14.0
-sphinxcontrib-svg2pdfconverter==0.1.0
-astroid
-sphinx-autoapi
-isort
-black
-readthedocs-sphinx-search

requirements-doc.txt

@@ -8,7 +8,7 @@ astroid
 setuptools
 
 # For sphinx
-Sphinx<4
+sphinx>=4.0.0
 sphinx-autoapi
 sphinx-rtd-theme
 sphinxcontrib-svg2pdfconverter