DOC, MAINT: Bump CircleCI Python version to 3.11 (scipy#19504)

* DOC, MAINT: Bump CircleCI Python version to 3.11

* Add pin for myst-nb on CircleCI config to fix doc build issue

[skip azp] [skip cirrus]

.circleci/config.yml

@@ -5,7 +5,7 @@ _defaults: &defaults
   docker:
     # CircleCI maintains a library of pre-built images
     # documented at https://circleci.com/docs/2.0/circleci-images/
-    - image: cimg/python:3.9
+    - image: cimg/python:3.11
   working_directory: ~/repo
 
 commands:
@@ -77,6 +77,7 @@ jobs:
             pip install cython
             pip install numpy==1.23.5
             pip install -r doc_requirements.txt
+            pip install "myst-nb<1.0.0"
             # `asv` pin because of slowdowns reported in gh-15568
             pip install mpmath gmpy2 "asv==0.4.2" pythran ninja meson click rich-click doit pydevtool pooch
             pip install pybind11
@@ -118,7 +119,7 @@ jobs:
           name: build docs
           no_output_timeout: 25m
           command: |
-            export PYTHONPATH=$PWD/build-install/lib/python3.9/site-packages
+            export PYTHONPATH=$PWD/build-install/lib/python3.11/site-packages
             python dev.py --no-build doc -j2
 
       - store_artifacts:
@@ -145,7 +146,7 @@ jobs:
           name: run asv
           no_output_timeout: 30m
           command: |
-            export PYTHONPATH=$PWD/build-install/lib/python3.9/site-packages
+            export PYTHONPATH=$PWD/build-install/lib/python3.11/site-packages
             cd benchmarks
             asv machine --machine CircleCI
             export SCIPY_GLOBAL_BENCH_NUMTRIALS=1
@@ -173,7 +174,7 @@ jobs:
           no_output_timeout: 25m
           command: |
             sudo apt-get install -y wamerican-small
-            export PYTHONPATH=$PWD/build-install/lib/python3.9/site-packages
+            export PYTHONPATH=$PWD/build-install/lib/python3.11/site-packages
             python dev.py --no-build refguide-check
 
 # Upload build output to scipy/devdocs repository, using SSH deploy keys.