remove 3.9 from testing matrix

.github/workflows/ci.yaml

@@ -18,7 +18,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-20.04, ubuntu-22.04]
-        python-version: ["3.9", "3.10"]
+        python-version: ["3.10"]
 
     steps:
       - name: Set up Python ${{ matrix.python-version }}
@@ -77,10 +77,10 @@ jobs:
     # Run on tagged push (generated by tbump).
     if: github.event_name == 'push' && startsWith(github.ref, 'refs/tags')
     steps:
-      - name: Set up Python 3.9
+      - name: Set up Python 3.10
         uses: actions/setup-python@v4
         with:
-          python-version: 3.9
+          python-version: 3.10
 
       - name: Install poetry
         uses: abatilo/actions-poetry@v2