Incorporate the Python version into keys for pip and pre-commit caches.

This should resolve the issue seen when the Python version changes before there
is an update to .pre-commit-config.yml which results in pre-commit pointing to
a non-existent Python installation.

.github/workflows/build.yml

@@ -14,20 +14,25 @@ jobs:
       - uses: actions/setup-python@v1
         with:
           python-version: 3.8
+      - name: Store installed Python version
+        run: |
+          echo "::set-env name=PY_VERSION::"\
+          "$(python -c "import platform;print(platform.python_version())")"
       - name: Cache pip test requirements
         uses: actions/cache@v1
         with:
           path: ~/.cache/pip
-          key: "${{ runner.os }}-pip-test-\
+          key: "${{ runner.os }}-pip-test-py${{ env.PY_VERSION }}-\
             ${{ hashFiles('**/requirements-test.txt') }}"
           restore-keys: |
+            ${{ runner.os }}-pip-test-py${{ env.PY_VERSION }}-
             ${{ runner.os }}-pip-test-
             ${{ runner.os }}-pip-
       - name: Cache pre-commit hooks
         uses: actions/cache@v1
         with:
           path: ~/.cache/pre-commit
-          key: "${{ runner.os }}-pre-commit-\
+          key: "${{ runner.os }}-pre-commit-py${{ env.PY_VERSION }}-\
             ${{ hashFiles('**/.pre-commit-config.yaml') }}"
       - name: Install dependencies
         run: |