fix: use conditional import for NotRequired to support Python 3.11+

Fixes #124

The issue was that `typing_extensions.NotRequired` was always imported,
but the dependency only installed typing_extensions for Python < 3.11.
In Python 3.11+, NotRequired is available in the standard typing module.

Changes:
- Added conditional import: use `typing.NotRequired` for Python 3.11+
- Falls back to `typing_extensions.NotRequired` for Python 3.10
- This eliminates the ModuleNotFoundError on Python 3.11+ environments

🤖 Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

src/claude_agent_sdk/types.py

@@ -6,7 +6,11 @@ from dataclasses import dataclass, field
 from pathlib import Path
 from typing import TYPE_CHECKING, Any, Literal, TypedDict
 
-from typing_extensions import NotRequired
+# NotRequired was added to typing in Python 3.11
+if sys.version_info >= (3, 11):
+    from typing import NotRequired
+else:
+    from typing_extensions import NotRequired
 
 if TYPE_CHECKING:
     from mcp.server import Server as McpServer