ci: Add matplotlib nightly wheels to HEAD of dependencies testing (#1835

)

* With the addition of matplotlib nightly wheels on the
scipy-wheels-nightly Anaconda Cloud organization package index,
wheels that are either at, or very close to, matplotlib's HEAD
on GitHub can now be tested. As the only place that matplotlib
is used in pyhf is in pyhf.contrib, only test the contrib module
instead of all of pyhf.
   - c.f. https://anaconda.org/scipy-wheels-nightly

.github/workflows/dependencies-head.yml

@@ -114,6 +114,42 @@ jobs:
       run: |
         pytest --ignore tests/benchmarks/ --ignore tests/contrib --ignore tests/test_notebooks.py
 
+  matplotlib:
+
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [ubuntu-latest]
+        python-version: ['3.10']
+
+    steps:
+    - uses: actions/checkout@v3
+
+    - name: Set up Python ${{ matrix.python-version }}
+      uses: actions/setup-python@v3
+      with:
+        python-version: ${{ matrix.python-version }}
+
+    - name: Install dependencies
+      run: |
+        python -m pip install --upgrade pip setuptools wheel
+        python -m pip --no-cache-dir --quiet install --upgrade .[test]
+        python -m pip uninstall --yes matplotlib
+        # Need to use --extra-index-url as dependencies aren't on scipy-wheels-nightly package index.
+        # Need to use --pre as dev releases will need priority over stable releases.
+        python -m pip install \
+          --upgrade \
+          --pre \
+          --extra-index-url https://pypi.anaconda.org/scipy-wheels-nightly/simple \
+          matplotlib
+
+    - name: List installed Python packages
+      run: python -m pip list
+
+    - name: Test contrib module API with pytest
+      run: |
+        pytest tests/contrib
+
   pytest:
 
     runs-on: ${{ matrix.os }}