INTPYTHON-841 Remove py39 fallback from typing.TypeAlias (#282)

INTPYTHON-841

## Summary

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One line change to remove fallback to get TypeAlias from
typing_extensions for python 3.9.

## Checklist

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### Checklist for Author

- [X] Did you update the changelog (if necessary)?
- [X] Is the intention of the code captured in relevant tests?
- [X] If there are new TODOs, has a related JIRA ticket been created?
- [X] Has a MongoDB Employee run [the patch build of this
PR](https://github.com/mongodb-labs/ai-ml-pipeline-testing?tab=readme-ov-file#running-a-patch-build-of-a-given-pr)?

### Checklist for Reviewer {@primary_reviewer}

- [ ] Does the title of the PR reference a JIRA Ticket?
- [ ] Do you fully understand the implementation? (Would you be
comfortable explaining how this code works to someone else?)
- [ ] Have you checked for spelling & grammar errors?
- [ ] Is all relevant documentation (README or docstring) updated?

## Focus Areas for Reviewer (optional)

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[Evergreen Patch
Link](https://spruce.mongodb.com/version/692f61ec37f69f0007aa54e6)

Author: caseyclements

libs/langchain-mongodb/langchain_mongodb/graphrag/graph.py

@@ -3,7 +3,7 @@
 import json
 import logging
 from copy import deepcopy
-from typing import TYPE_CHECKING, Any, Callable, Dict, List, Optional, Union
+from typing import TYPE_CHECKING, Any, Callable, Dict, List, Optional, TypeAlias, Union
 
 from langchain_core.documents import Document
 from langchain_core.language_models.chat_models import BaseChatModel
@@ -21,11 +21,6 @@
 from .schema import entity_schema
 
 if TYPE_CHECKING:
-    try:
-        from typing import TypeAlias  # type:ignore[attr-defined] # Python 3.10+
-    except ImportError:  # Python 3.9 fallback  # TODO Remove [PYTHON-5658]
-        from typing_extensions import TypeAlias
-
     Entity: TypeAlias = Dict[str, Any]
     """Represents an Entity in the knowledge graph with specific schema. See .schema"""