ci: install Qt binding in compile-translations job

Author: PierreRaybaut

.github/workflows/_build.yml

@@ -110,6 +110,9 @@ jobs:
   compile-translations:
     needs: preflight
     runs-on: ubuntu-latest
+    env:
+      DISPLAY: ":99.0"
+      QT_QPA_PLATFORM: offscreen
     steps:
       - uses: actions/checkout@v4
         with:
@@ -121,11 +124,23 @@ jobs:
           python-version: ${{ inputs.python-version }}
           cache: pip
 
-      - name: Install minimal Python dependencies (sphinx + guidata)
+      # `sphinx -b gettext` parses doc/conf.py which transitively imports
+      # `datalab.*` -> `qtpy`. Without a Qt binding installed `qtpy` raises
+      # `QtBindingsNotFoundError` and the build aborts. Installing PyQt5
+      # (no graphical session required thanks to QT_QPA_PLATFORM=offscreen)
+      # is enough to let imports succeed.
+      - name: Install system dependencies (Qt offscreen)
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y --no-install-recommends \
+            libxkbcommon-x11-0 libxcb-icccm4 libxcb-image0 libxcb-keysyms1 \
+            libxcb-randr0 libxcb-render-util0 libxcb-xinerama0 libxcb-xfixes0
+
+      - name: Install Python dependencies
         run: |
           python -m pip install --upgrade pip
-          python -m pip install --upgrade guidata
-          python -m pip install .[doc]
+          python -m pip install --upgrade PythonQwt guidata PlotPy Sigima
+          python -m pip install .[doc,qt]
 
       - name: Compile docs gettext + sphinx_intl + UI .mo
         run: |