docs: update Sphinx version on RTD (#3882)

python3.9 -mvirtualenv sphinx
    . sphinx/bin/activate
    pip install Sphinx
    pip freeze > requirements.txt

.readthedocs.yaml

@@ -5,7 +5,10 @@ sphinx:
   configuration: null
   fail_on_warning: false
 
+build:
+  tools:
+    python: "3.9"
+
 python:
-  version: 3.8
   install:
     - requirements: docs/requirements.txt

docs/requirements.txt

@@ -1,42 +1,27 @@
 # primary
-Sphinx==3.5.4
+sphinx==6.1.3
 
 # dependencies
-alabaster==0.7.12
+alabaster==0.7.13
-appdirs==1.4.3
+Babel==2.11.0
-Babel==2.9.0
+certifi==2022.12.7
-CacheControl==0.12.6
+charset-normalizer==3.0.1
-certifi==2019.11.28
+docutils==0.19
-chardet==3.0.4
+idna==3.4
-colorama==0.4.3
+imagesize==1.4.1
-contextlib2==0.6.0
+importlib-metadata==6.0.0
-distlib==0.3.0
+Jinja2==3.1.2
-distro==1.4.0
+MarkupSafe==2.1.2
-docutils==0.16
+packaging==23.0
-html5lib==1.0.1
+Pygments==2.14.0
-idna==2.8
+pytz==2022.7.1
-imagesize==1.2.0
+requests==2.28.2
-ipaddr==2.2.0
+snowballstemmer==2.2.0
-Jinja2==2.11.3
+sphinxcontrib-applehelp==1.0.3
-lockfile==0.12.2
-MarkupSafe==1.1.1
-msgpack==0.6.2
-packaging==20.3
-pep517==0.8.2
-progress==1.5
-Pygments==2.8.1
-pyparsing==2.4.6
-pytoml==0.1.21
-pytz==2021.1
-requests==2.22.0
-retrying==1.3.3
-six==1.14.0
-snowballstemmer==2.1.0
-sphinxcontrib-applehelp==1.0.2
 sphinxcontrib-devhelp==1.0.2
-sphinxcontrib-htmlhelp==1.0.3
+sphinxcontrib-htmlhelp==2.0.0
 sphinxcontrib-jsmath==1.0.1
 sphinxcontrib-qthelp==1.0.3
-sphinxcontrib-serializinghtml==1.1.4
+sphinxcontrib-serializinghtml==1.1.5
-urllib3==1.25.8
+urllib3==1.26.14
-webencodings==0.5.1
+zipp==3.11.0