⬆️🪝 Update pre-commit hook adamchainz/blacken-docs to v1.20.0 (#455)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
|
[adamchainz/blacken-docs](https://redirect.github.com/adamchainz/blacken-docs)
| repository | minor | `1.19.1` -> `1.20.0` |

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### Release Notes

<details>
<summary>adamchainz/blacken-docs (adamchainz/blacken-docs)</summary>

###
[`v1.20.0`](https://redirect.github.com/adamchainz/blacken-docs/blob/HEAD/CHANGELOG.rst#1200-2025-09-08)

[Compare
Source](https://redirect.github.com/adamchainz/blacken-docs/compare/1.19.1...1.20.0)

- Support Python 3.14.

</details>

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.pre-commit-config.yaml

@@ -65,7 +65,7 @@ repos:
 
   # Also run Black on examples in the documentation
   - repo: https://github.com/adamchainz/blacken-docs
-    rev: 1.19.1
+    rev: 1.20.0
     hooks:
       - id: blacken-docs
         additional_dependencies: [black==25.*]