CICD: fix PyPy version

The error from CI was:

    Run actions/setup-python@v2
      with:
        python-version: pypy3
        token: ***
    Error: PyPy 3.6 not found

.github/workflows/cicd.yml

@@ -16,13 +16,15 @@ jobs:
         - 3.9  # Reference version used to run integration tests;
                # to keep in sync with 'if: ' statements in the rest
                # of this file.
-        - pypy3
+        - pypy3.8  # To keep in sync with other pypy releases referenced in the
+                   # "Docker image tests" and "Push Docker image to Docker Hub"
+                   # sections of this file.
 
     steps:
       - uses: actions/checkout@v2
 
       - name: Set up Python ${{ matrix.python-version }}
-        uses: actions/setup-python@v2
+        uses: actions/setup-python@v4
         with:
           python-version: ${{ matrix.python-version }}
 
@@ -371,7 +373,7 @@ jobs:
       - uses: actions/checkout@v2
 
       - name: Set up Python ${{ matrix.python-version }}
-        uses: actions/setup-python@v2
+        uses: actions/setup-python@v4
         with:
           python-version: 3.6