feat: 支持 Python 3.8+ (0.3.0)

- requires-python >=3.8, classifiers 增加 3.8/3.9
- 代码经 ast feature_version=(3,8) 校验, 无 3.9+ 专属语法; str|None 等注解由 future annotations 延迟求值
- CI 测试/构建/smoke 矩阵扩展为 3.8-3.14, 裸资产数 6→8
- 注意: Windows runner 需预装 Python 3.8 与 3.9 (py launcher)

tests/test_dlp_io_packaging.py

@@ -42,7 +42,7 @@ class PackageClassifier(Enum):
 
 class WorkflowStep(Enum):
     CLONE_TAG = "Clone immutable package tag"
-    TEST_MATRIX = "Test Python 3.10-3.14"
+    TEST_MATRIX = "Test Python 3.8-3.14"
     BUILD = "Build and check distribution"
     SMOKE = "Smoke test release artifacts"
     RELEASE = "Create Gitea release with wheels"
@@ -108,7 +108,7 @@ def test_pyproject_declares_structured_public_package_contract() -> None:
     project = config["project"]
 
     assert PackageName(canonicalize_name(project["name"])) is PackageName.DLP_IO
-    assert SpecifierSet(project["requires-python"]) == SpecifierSet(">=3.10")
+    assert SpecifierSet(project["requires-python"]) == SpecifierSet(">=3.8")
     assert {DynamicField(item) for item in project["dynamic"]} == {
         DynamicField.VERSION
     }