workflows: test-matplotlib: add python3-tk to OS dependencies list

Signed-off-by: Trevor Gamblin <tgamblin@baylibre.com>

Author: threexc

.github/workflows/test-matplotlib.yml

@@ -74,6 +74,8 @@ jobs:
         run: |
           git apply python-wheels/patches/matplotlib/${{ env.MPL_VERSION }}/00*.patch
 
+      # add python3-tk to system install to avoid "No module named 'tkinter'"
+      # failures when testing matplotlib backends
       - name: Install OS dependencies
         run: |
           echo 'Acquire::Retries "3";' | sudo tee /etc/apt/apt.conf.d/80-retries
@@ -108,6 +110,7 @@ jobs:
             lmodern \
             ninja-build \
             pkg-config \
+            python3-tk \
             qtbase5-dev \
             texlive-fonts-recommended \
             texlive-latex-base \