ci: bump Python version in reusable_testing

GitHub images with Ubuntu 24.04 have no longer Python 3.7, see [1].
This patch updates the version of Python to 3.9 to be consistent with
other workflows.

[1]: https://raw.githubusercontent.com/actions/python-versions/main/versions-manifest.json

Needed for tarantool/tarantool#11220

Author: Buristan

.github/workflows/reusable_testing.yml

@@ -34,7 +34,7 @@ jobs:
       - name: Setup python3 for tests
         uses: actions/setup-python@v5
         with:
-          python-version: 3.7
+          python-version: 3.9
 
       - name: Install build requirements
         run: sudo apt-get -y install libsasl2-dev