Use workflow variable to set Python version in "Check Python" workflow

This will make version bumps easy, especially as additional jobs are added to the workflow

.github/workflows/check-python-task.yml

@@ -1,5 +1,9 @@
 name: Check Python
 
+env:
+  # See: https://github.com/actions/setup-python/tree/v2#available-versions-of-python
+  PYTHON_VERSION: "3.8"
+
 on:
   push:
     paths:
@@ -31,7 +35,7 @@ jobs:
       - name: Install Python
         uses: actions/setup-python@v2
         with:
-          python-version: "3.8"
+          python-version: ${{ env.PYTHON_VERSION }}
 
       - name: Install Poetry
         run: pip install poetry