Update the min python version to 3.9

Both jupyterlab/maintainer-tools and jupyter-server dropped support for 3.8 back in December of last year.

Author: ojarjur

.github/workflows/build.yml

@@ -12,12 +12,10 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest, macos-latest]
-        python-version: ["3.8", "3.11"]
+        python-version: ["3.9", "3.11", "3.12"]
         include:
           - os: ubuntu-latest
-            python-version: "3.9"
-          - os: ubuntu-latest
-            python-version: "pypy-3.8"
+            python-version: "pypy-3.9"
           - os: macos-latest
             python-version: "3.10"
     steps: