Update ci.yaml

We can keep testing with Python `3.7` by using `ubuntu-22.04` instead of `ubuntu-latest` according to the `setup-python` issue.

More info: https://github.com/actions/setup-python/issues/962

.github/workflows/ci.yaml

@@ -11,11 +11,11 @@ on:
 
 jobs:
   test:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     strategy:
       fail-fast: false
       matrix:
-        python-version: [3.8, 3.9, 3.10, 3.11, 3.12, 3.13]
+        python-version: [3.7, 3.8, 3.9, 3.10, 3.11, 3.12, 3.13]
     steps:
       - uses: actions/checkout@v4
       - name: Install Python ${{ matrix.python-version }}