build: upgrade Sphinx 4→9, drop m2r2/recommonmark, Python 3.11+

- Upgrade sphinx 4.x → >=7.0 (resolves Python 3.13 incompatibility from
  imghdr removal, closes #157)
- Replace m2r2 + recommonmark with myst-parser >=2.0 (handles all Markdown;
  m2r2/recommonmark are unmaintained and incompatible with Sphinx 7+)
- Drop setuptools workaround (was needed only for m2r2)
- Pin python >=3.11 (matching .readthedocs.yml; 3.13 in CI/RTD)
- Bump sphinx_rtd_theme to >=2.0, sphinx-tabs to ^3.4, sphinx-click to >=6.0
- Update .readthedocs.yml: python 3.11 → 3.13
- Update CI: actions/checkout@v2→v4, setup-python@v1→v5, python 3.12→3.13
- Regenerate poetry.lock (sphinx 9.0.4, myst-parser 5.0.0)

Author: TimeToBuildBob

.github/workflows/build.yml

@@ -10,14 +10,14 @@ jobs:
   build:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
       with:
           submodules: recursive
 
     - name: Set up Python
-      uses: actions/setup-python@v1
+      uses: actions/setup-python@v5
       with:
-        python-version: 3.12
+        python-version: "3.13"
     - name: Install APT dependencies
       run: |
         sudo apt-get install -y git build-essential apt-utils wget libfreetype6 libpng-dev libopenblas-dev gcc gfortran libsnappy-dev graphviz

.readthedocs.yml

@@ -8,7 +8,7 @@ version: 2
 build:
   os: ubuntu-22.04
   tools:
-    python: "3.11"
+    python: "3.13"
   jobs:
     post_create_environment:
       - pip install poetry