test workflow: Upgrade Python version to 3.10.

This is an optional commit that upgrade the Python
version for github workflows when running tests.
This commit doesn't try to drop support for
Python 3.8.

.github/workflows/zulip-tests.yml

@@ -13,10 +13,10 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      - name: Set up Python 3.8
+      - name: Set up Python 3.10
         uses: actions/setup-python@v5
         with:
-          python-version: "3.8"
+          python-version: "3.10"
 
       - name: Install dependencies
         run: tools/provision --force