update actions/checkout action to v4 (arska#19)

Cherry picked from Commit: 132df1f [132df1f]

.github/workflows/main.yml

@@ -10,12 +10,12 @@ jobs:
     name: "run tox"
     strategy:
       matrix:
-        python-version: [3.6, 3.7, 3.8, 3.9, pypy3]
+        python-version: [3.8, 3.9, "3.10", 3.11]
 
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
     - name: Set up Python ${{ matrix.python-version }}
-      uses: actions/setup-python@v2
+      uses: actions/setup-python@v4
       with:
         python-version: ${{ matrix.python-version }}
     - name: Install dependencies
@@ -32,10 +32,10 @@ jobs:
     runs-on: "ubuntu-latest"
     needs: [test]
     steps:
-      - uses: "actions/checkout@v2"
-      - uses: "actions/setup-python@v2"
+      - uses: "actions/checkout@v4"
+      - uses: "actions/setup-python@v4"
         with:
-          python-version: "3.8"
+          python-version: "3.11"
 
       - name: "Install pep517 and twine"
         run: "python -m pip install pep517 twine"