chore: switch back to ubuntu-latest (#4065)

We previously pinned this workflow to `ubuntu-20.04` to preserve Python 2.7 compatibility, but have since dropped Python 2.7 so can now unpin.

Refs: #3086
Refs: #3093
Refs: actions/runner-images#11101

Author: richardlau

.github/workflows/push.yml

@@ -6,13 +6,13 @@ on:
   pull_request:
 jobs:
   Python:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v4
-    - name: Set up Python 3.7
+    - name: Set up Python 3.9
       uses: actions/setup-python@v5
       with:
-        python-version: 3.7
+        python-version: 3.9
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip