Add test_matrix workflow.

.github/workflows/test-matrix.yml

@@ -0,0 +1,59 @@
+# Run a matrix of tests across different platforms and python versions.
+# This is run on each pull request so we get a complete picture of any
+# platform-specific issues not caught earlier.
+#
+# See also:
+#    unit-test.yml: run on a single platform every time a branch is pushed
+#    coverage-report.yml: runs on merge to main to generate coverage reports
+
+# Based on https://github.com/marketplace/actions/install-poetry-action#running-on-windows
+
+name: test-matrix
+
+on: pull_request
+
+jobs:
+  test-matrix:
+    strategy:
+      matrix:
+        python-version: ["3.11", "3.12"]
+        os: [ "ubuntu-latest", "macos-latest", "windows-latest" ]
+      fail-fast: false
+    defaults:
+      run:
+        shell: bash
+    runs-on: ${{ matrix.os }}
+    steps:
+      - name: Check out repository
+        uses: actions/checkout@v4
+      - name: Set up python
+        id: setup-python
+        uses: actions/setup-python@v5
+        with:
+          python-version: ${{ matrix.python-version }}
+      - name: Install Poetry
+        uses: snok/install-poetry@v1
+        with:
+          virtualenvs-create: true
+          virtualenvs-in-project: true
+      - name: Load cached venv
+        id: cached-pip-wheels
+        uses: actions/cache@v4
+        with:
+          path: ~/.cache
+          key: venv-${{ runner.os }}-${{ steps.setup-python.outputs.python-version }}-${{ hashFiles('**/poetry.lock') }}
+      - name: Install dependencies
+        run: poetry install --no-interaction --no-root
+      - name: Install library
+        run: poetry install --no-interaction
+      - name: Run tests
+        run: |
+          source $VENV
+          pytest
+      - name: Archive test artifacts
+        if: always()
+        uses: actions/upload-artifact@v4
+        with:
+          name: test_artifacts-${{ matrix.os }}-${{ matrix.python-version }}
+          path: tests/_test_artifacts
+          retention-days: 7