ci: drop Python 3.9 from test matrix

msgpack 1.2.1 and urllib3 2.7.0 (the only versions that fix CTO-4990 and
CTO-4807) require Python >=3.10. Runtime image is python:3.11, so 3.9 was
CI-only. Dropping it unblocks the security bumps.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: KrishnaSuravarapu

.github/workflows/unit-tests.yml

@@ -5,7 +5,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: ['3.9', '3.10', '3.11']
+        python-version: ['3.10', '3.11']
     steps:
     - uses: actions/checkout@v3
     - name: Set up Python ${{ matrix.python-version }}