[pyproject.toml] add `tiktoken` when install `langchain[openai]` (#4514)

# Add `tiktoken` as dependency when installed as `langchain[openai]`

Fixes #4513 (issue)

## Who can review?

Community members can review the PR once tests pass. Tag
maintainers/contributors who might be interested:

@vowelparrot 

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pyproject.toml

@@ -160,7 +160,7 @@ setuptools = "^67.6.1"
 [tool.poetry.extras]
 llms = ["anthropic", "cohere", "openai", "nlpcloud", "huggingface_hub", "manifest-ml", "torch", "transformers"]
 qdrant = ["qdrant-client"]
-openai = ["openai"]
+openai = ["openai", "tiktoken"]
 cohere = ["cohere"]
 in_memory_store = ["docarray"]
 hnswlib = ["docarray", "protobuf", "hnswlib"]