Bump tiktoken from 0.4.0 to 0.5.1 in /platform (#1308)

Bumps [tiktoken](https://github.com/openai/tiktoken) from 0.4.0 to 0.5.1.
- [Release notes](https://github.com/openai/tiktoken/releases)
- [Changelog](https://github.com/openai/tiktoken/blob/main/CHANGELOG.md)
- [Commits](openai/tiktoken@0.4.0...0.5.1)

---
updated-dependencies:
- dependency-name: tiktoken
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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