ci: Upgrade setup-python to fix GitHub Actions test phase (#564)

Author: jamesholland-uk

.github/workflows/ci.yml

@@ -23,7 +23,7 @@ jobs:
       - uses: actions/checkout@v2
 
       - name: Set up Python ${{ matrix.python-version }}
-        uses: actions/setup-python@v1
+        uses: actions/setup-python@v2
         with:
           python-version: ${{ matrix.python-version }}