Bump Python 3.8 -> 3.9 in {pr,push}-checks.yml GH workflows

Closes #893

.github/workflows/pr-checks.yml

@@ -45,10 +45,10 @@ jobs:
     ####################################################################
     # Get Python set up
     ####################################################################
-    - name: Set up Python 3.8
+    - name: Set up Python 3.9
       uses: actions/setup-python@v2
       with:
-        python-version: 3.8
+        python-version: 3.9
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip

.github/workflows/push-checks.yml

@@ -20,10 +20,10 @@ jobs:
     - uses: actions/checkout@v2
       with:
         fetch-depth: 0
-    - name: Set up Python 3.8
+    - name: Set up Python 3.9
       uses: actions/setup-python@v2
       with:
-        python-version: 3.8
+        python-version: 3.9
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip