chore(deps): update dependency pg8000 to v1.16.4 (GoogleCloudPlatform#4429)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [pg8000](https://togithub.com/tlocke/pg8000) | patch | `==1.16.3` -> `==1.16.4` |

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### Release Notes

<details>
<summary>tlocke/pg8000</summary>

### [`v1.16.4`](https://togithub.com/tlocke/pg8000/compare/1.16.3...1.16.4)

[Compare Source](https://togithub.com/tlocke/pg8000/compare/1.16.3...1.16.4)

</details>

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cloud-sql/postgres/sqlalchemy/requirements.txt

@@ -1,4 +1,4 @@
 Flask==1.1.2
 SQLAlchemy==1.3.18
-pg8000==1.16.3
+pg8000==1.16.4
 gunicorn==20.0.4