chore(deps): update pre-commit hook asottile/pyupgrade to v3.21.0 (#571)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [asottile/pyupgrade](https://redirect.github.com/asottile/pyupgrade) |
repository | minor | `v3.20.0` -> `v3.21.0` |

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---

### Release Notes

<details>
<summary>asottile/pyupgrade (asottile/pyupgrade)</summary>

###
[`v3.21.0`](https://redirect.github.com/asottile/pyupgrade/compare/v3.20.0...v3.21.0)

[Compare
Source](https://redirect.github.com/asottile/pyupgrade/compare/v3.20.0...v3.21.0)

</details>

---

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Automerge - At any time (no schedule defined).

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---

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this box

---

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.pre-commit-config.yaml

@@ -29,7 +29,7 @@ repos:
         files: \.(md|ya?ml|js|css)$
 
   - repo: https://github.com/asottile/pyupgrade
-    rev: v3.20.0
+    rev: v3.21.0
     hooks:
       - id: pyupgrade
         args: [--py39-plus]