Update tests.yml to run python 3.7 tests on legacy version macOS

Thanks to actions/runner-images#9770 (comment)

.github/workflows/tests.yml

@@ -15,6 +15,12 @@ jobs:
       matrix:
         os: [ubuntu-latest, macos-latest]
         python-version: ["3.7", "3.10"]
+        exclude:  # Python < v3.8 does not support Apple Silicon ARM64.
+          - python-version: "3.7"
+            os: macos-latest
+        include:  # So run those legacy versions on Intel CPUs.
+          - python-version: "3.7"
+            os: macos-13
     steps:
       - uses: actions/checkout@v3
       - name: Set up Python ${{ matrix.python-version }}
@@ -36,4 +42,4 @@ jobs:
       - name: Run tests
         run: |
           cd tests
-          pytest
+          pytest