CI: Add CI to test matplotlib against free-threaded Python

Co-authored-by: Edgar Andrés Margffoy Tuay <andfoy@gmail.com>

Author: QuLogic

.github/workflows/tests.yml

@@ -91,6 +91,13 @@ jobs:
             pyqt6-ver: '!=6.6.0'
             # https://bugreports.qt.io/projects/PYSIDE/issues/PYSIDE-2346
             pyside6-ver: '!=6.5.1'
+          - name-suffix: "Free-threaded"
+            os: ubuntu-22.04
+            python-version: '3.13t'
+            # https://www.riverbankcomputing.com/pipermail/pyqt/2023-November/045606.html
+            pyqt6-ver: '!=6.6.0'
+            # https://bugreports.qt.io/projects/PYSIDE/issues/PYSIDE-2346
+            pyside6-ver: '!=6.5.1'
           - os: macos-12  # This runner is on Intel chips.
             python-version: '3.10'
             # https://bugreports.qt.io/projects/PYSIDE/issues/PYSIDE-2346
@@ -111,10 +118,18 @@ jobs:
 
       - name: Set up Python ${{ matrix.python-version }}
         uses: actions/setup-python@v5
+        if: matrix.python-version != '3.13t'
         with:
           python-version: ${{ matrix.python-version }}
           allow-prereleases: true
 
+      - name: Set up Python ${{ matrix.python-version }}
+        uses: deadsnakes/action@6c8b9b82fe0b4344f4b98f2775fcc395df45e494  # v3.1.0
+        if: matrix.python-version == '3.13t'
+        with:
+          python-version: '3.13'
+          nogil: true
+
       - name: Install OS dependencies
         run: |
           case "${{ runner.os }}" in
@@ -160,6 +175,11 @@ jobs:
               texlive-luatex \
               texlive-pictures \
               texlive-xetex
+            if [[ "${{ matrix.python-version }}" = '3.13t' ]]; then
+              # TODO: Remove this once setup-python supports nogil distributions.
+              sudo apt-get install -yy --no-install-recommends \
+                python3.13-tk-nogil
+            fi
             if [[ "${{ matrix.os }}" = ubuntu-20.04 ]]; then
               sudo apt-get install -yy --no-install-recommends libopengl0
             else  # ubuntu-22.04
@@ -318,6 +338,9 @@ jobs:
 
       - name: Run pytest
         run: |
+          if [[ "${{ matrix.python-version }}" == '3.13t' ]]; then
+            export PYTHON_GIL=0
+          fi
           pytest -rfEsXR -n auto \
             --maxfail=50 --timeout=300 --durations=25 \
             --cov-report=xml --cov=lib --log-level=DEBUG --color=yes