[BUG] Ensure compatibility with older Python versions

Added a fallback import for `NotRequired` from `typing_extensions` to maintain compatibility with Python versions that lack native support for this typing feature.

Author: Leguark

gempy/modules/json_io/schema.py

@@ -3,7 +3,13 @@
 This module defines the expected structure of JSON files for loading and saving GemPy models.
 """
 
-from typing import TypedDict, List, Dict, Any, Optional, Union, Sequence, NotRequired
+from typing import TypedDict, List, Dict, Any, Optional, Union, Sequence
+
+try:
+    from typing import NotRequired
+except ImportError:
+    from typing_extensions import NotRequired # Fallback for older Python versions
+
 
 class SurfacePoint(TypedDict):
     x: float