chore(deps): update dependency tiktoken to v0.5.1

[renovate[bot]]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
tiktoken (changelog)	==0.4.0 -> ==0.5.1

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Release Notes

openai/tiktoken (tiktoken)

### [`v0.5.1`](https://togithub.com/openai/tiktoken/blob/HEAD/CHANGELOG.md#v051) [Compare Source](https://togithub.com/openai/tiktoken/compare/0.5.0...0.5.1) - Add `encoding_name_for_model`, undo some renames to variables that are implementation details ### [`v0.5.0`](https://togithub.com/openai/tiktoken/blob/HEAD/CHANGELOG.md#v050) [Compare Source](https://togithub.com/openai/tiktoken/compare/0.4.0...0.5.0) - Add `tiktoken._educational` submodule to better document how byte pair encoding works - Ensure `encoding_for_model` knows about several new models - Add `decode_with_offets` - Better error for failures with the plugin mechanism - Make more tests public - Update versions of dependencies

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