deps!: drop Python 3.9 support to allow urllib3 >=2.7.0

urllib3 2.7.0 dropped Python 3.9, which blocked transitive updates while
the project still declared python = ">=3.9,<4". Python 3.9 reached EOL on
2025-10-31, so this raises the floor to Python 3.10.

BREAKING CHANGE: Python 3.9 is no longer supported. Minimum supported
version is now Python 3.10.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: matthewelwell

.github/workflows/pytest.yml

@@ -13,7 +13,7 @@ jobs:
     strategy:
       max-parallel: 5
       matrix:
-        python-version: ['3.9', '3.10', '3.11', '3.12', '3.13']
+        python-version: ['3.10', '3.11', '3.12', '3.13']
 
     steps:
       - name: Cloning repo