Use workflow variables for tool versions

Placement of this information at the top of the file, where it is easy to find and edit, facilitates updates to the
workflows as the tool versions used for project development are bumped periodically.

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

@@ -1,5 +1,9 @@
 name: Spell Check
 
+env:
+  # See: https://github.com/actions/setup-python/tree/v2#available-versions-of-python
+  PYTHON_VERSION: "3.9"
+
 # See: https://docs.github.com/en/free-pro-team@latest/actions/reference/events-that-trigger-workflows
 on:
   push:
@@ -21,7 +25,7 @@ jobs:
       - name: Install Python
         uses: actions/setup-python@v2
         with:
-          python-version: "3.9"
+          python-version: ${{ env.PYTHON_VERSION }}
 
       - name: Install Poetry
         run: pip install poetry