test workflow: Upgrade Python version to 3.10.

This commit only upgrade the Python version for
github workflows when running tests. The previous
Python version of 3.8.18 have some outdated mypy
linting error over at the bots/slasesforce.py file

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