chore: drop Python 3.9, raise minimum version to 3.10 (#109)

### what

Raise minimum Python version from 3.9 to 3.10 in `pyproject.toml`,
remove 3.9 from the CI test matrix, and regenerate `uv.lock`.

### why

Python 3.9 reached end-of-life in October 2025. Dropping it removes
the need for split dependency resolution markers in the lock file,
eliminating Python-3.9-only wheels for packages such as `urllib3`,
`cffi`, `charset-normalizer`, `cfgv`, and others.

### testing

`uv lock` regenerated the lock file cleanly with no resolution
errors under the new constraint.

### docs

No user-facing or runbook changes needed. No release note required
for a minimum-version bump.

---------------------------

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Author: jtroup

.github/workflows/ci.yaml

@@ -29,7 +29,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: ["3.9", "3.10", "3.11", "3.12", "3.13", "3.14"]
+        python-version: ["3.10", "3.11", "3.12", "3.13", "3.14"]
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
       - uses: wistia/parse-tool-versions@32f568a4ffd4bfa7720ebf93f171597d1ebc979a # v2.1.1

pyproject.toml

@@ -7,7 +7,7 @@ name = "stacklet.client.platform"
 description = "Stacklet Platform Client"
 authors = [{ name = "Stacklet Engineering", email = "foss+stacklet-admin@stacklet.io" }]
 maintainers = [{ name = "Stacklet Engineering", email = "foss+stacklet-admin@stacklet.io" }]
-requires-python = ">=3.9,<=3.14"
+requires-python = ">=3.10,<=3.14"
 readme = "README.md"
 license = "Apache-2.0"
 dependencies = [