chore(deps): update dependency pyyaml to v5.4 (#735)

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This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [PyYAML](https://pyyaml.org/) ([source](https://togithub.com/yaml/pyyaml)) | minor | `==5.3.1` -> `==5.4` |

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### Release Notes

<details>
<summary>yaml/pyyaml</summary>

### [`v5.4`](https://togithub.com/yaml/pyyaml/compare/5.3.1...5.4)

[Compare Source](https://togithub.com/yaml/pyyaml/compare/5.3.1...5.4)

</details>

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requirements.txt

@@ -5,5 +5,5 @@ jinja2==2.11.2
 MarkupSafe==1.1.1
 protobuf==3.14.0
 pypandoc==1.5
-PyYAML==5.3.1
+PyYAML==5.4
 dataclasses==0.6  # TODO(busunkim) remove when 3.6 support is dropped