Update actions/setup-python to v6

Author: mwilliamson

.github/workflows/tests.yml

@@ -15,7 +15,7 @@ jobs:
     - uses: actions/checkout@v5
 
     - name: Use Python ${{ matrix.python-version }}
-      uses: actions/setup-python@v4
+      uses: actions/setup-python@v6
       with:
         python-version: ${{ matrix.python-version }}
 
@@ -35,7 +35,7 @@ jobs:
     - uses: actions/checkout@v5
 
     - name: Use Python ${{ matrix.python-version }}
-      uses: actions/setup-python@v4
+      uses: actions/setup-python@v6
       with:
         python-version: ${{ matrix.python-version }}