GitHub Actions: Upgrade actions/checkout

https://github.com/actions/checkout/releases
Signed-off-by: Christian Clauss <[email protected]>

Author: cclauss

.github/workflows/ci.yml

@@ -11,7 +11,7 @@ jobs:
                 python-version: [2.7, 3.5, 3.6, 3.7, 3.8, 3.9]
 
         steps:
-            - uses: actions/checkout@v1
+            - uses: actions/checkout@v2
             - name: Set up Python ${{ matrix.python-version }}
               uses: actions/setup-python@v2
               with: