Upgrade and pin Sphinx version

In future, we should ask "renovatebot" to update these version. I
don't have an opinion on whether to use "uv" or something else, but
I think we do want lockfiles, and I don't know of a natural way to
install Sphinx via Cargo.

No particular reason for this Python version.

Part of #11996

.github/actions/install-dependencies/action.yml

@@ -18,7 +18,6 @@ runs:
   steps:
     - shell: bash
       env:
-        include_sphinx: ${{ inputs.include_sphinx }}
         include_pcre: ${{ inputs.include_pcre }}
       run: |
         set -x
@@ -26,7 +25,6 @@ runs:
         sudo apt install \
           gettext \
           $(if $include_pcre; then echo libpcre2-dev; fi) \
-          $(if $include_sphinx; then echo python3-sphinx; fi) \
         ;
         : "system test dependencies"
         sudo apt install \
@@ -39,5 +37,5 @@ runs:
           tmux \
           wget \
         ;
-    - uses: ./.github/actions/install-sphinx-markdown-builder
+    - uses: ./.github/actions/install-sphinx
       if: ${{ inputs.include_sphinx == 'true' }}

.github/actions/install-sphinx-markdown-builder/action.yml

@@ -1,14 +0,0 @@
-name: Install sphinx-markdown-builder
-
-permissions:
-  contents: read
-
-runs:
-  using: "composite"
-  steps:
-    - shell: bash
-      run: |
-        set -x
-        commit=7c4daaf39a92f93deec948ecc1772e23bac44588
-        pip install git+https://github.com/liran-funaro/sphinx-markdown-builder@"$commit"
-        python -c 'import sphinx_markdown_builder'

.github/actions/install-sphinx/action.yml

@@ -0,0 +1,20 @@
+name: Install sphinx
+
+permissions:
+  contents: read
+
+runs:
+  using: "composite"
+  steps:
+    - shell: bash
+      run: |
+        set -x
+        sudo pip install uv --break-system-packages
+        # Check that pyproject.toml and the lock file are in sync.
+        # TODO Use "uv" to install Python as well.
+        : 'Note that --no-managed-python below would be implied but be explicit'
+        uv --no-managed-python lock --check
+        # Install globally.
+        sudo uv --no-managed-python pip install --group=dev --system --break-system-packages
+        # Smoke test.
+        python -c 'import sphinx; import sphinx_markdown_builder'

.github/workflows/release.yml

@@ -41,8 +41,8 @@ jobs:
           # Workaround for https://github.com/actions/checkout/issues/882
           ref: ${{ inputs.version }}
       - name: Install dependencies
-        run: sudo apt install cmake gettext ninja-build python3-pip python3-sphinx
-      - uses: ./.github/actions/install-sphinx-markdown-builder
+        run: sudo apt install cmake gettext ninja-build python3-pip
+      - uses: ./.github/actions/install-sphinx
       - name: Create tarball
         run: |
           set -x
@@ -80,7 +80,8 @@ jobs:
         with:
           targets: x86_64-unknown-linux-musl,aarch64-unknown-linux-musl
       - name: Install dependencies
-        run: sudo apt install crossbuild-essential-arm64 gettext musl-tools python3-sphinx
+        run: sudo apt install crossbuild-essential-arm64 gettext musl-tools
+      - uses: ./.github/actions/install-sphinx
       - name: Build statically-linked executables
         run: |
           set -x