release: 3.21.0 (#3987)

Automated Release PR
---


##
[3.21.0](https://github.com/openai/openai-python/compare/v3.20.0...v3.21.0)
(2026-09-29)


### Features

* **api:** add GPT-6.1 Sol model identifier
([#3986](https://github.com/openai/openai-python/issues/3986))
([160b2c0](https://github.com/openai/openai-python/commit/160b2c0230b9afac15862dcdf8a764e5bae18eb3))

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