Downgrade dependency for pg8000 (GoogleCloudPlatform#4914)

## Description

Fixes GoogleCloudPlatform#4854

Note: It's a good idea to open an issue first for discussion.

## Checklist
- [ ] I have followed [Sample Guidelines from AUTHORING_GUIDE.MD](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md)
- [ ] README is updated to include [all relevant information](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md#readme-file)
- [ ] **Tests** pass:   `nox -s py-3.6` (see [Test Environment Setup](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md#test-environment-setup))
- [ ] **Lint** pass:   `nox -s lint` (see [Test Environment Setup](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md#test-environment-setup))
- [ ] These samples need a new **API enabled** in testing projects to pass (let us know which ones)
- [ ] These samples need a new/updated **env vars** in testing projects set to pass (let us know which ones)
- [ ] Please **merge** this PR for me once it is approved.
- [ ] This sample adds a new sample directory, and I updated the [CODEOWNERS file](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/.github/CODEOWNERS) with the codeowners for this sample

Author: averikitsch

cloud-sql/postgres/sqlalchemy/requirements.txt

@@ -1,4 +1,4 @@
 Flask==1.1.2
 SQLAlchemy==1.3.20
-pg8000==1.16.6
+pg8000==1.16.5 # Do not upgrade >=1.6.6 until https://github.com/tlocke/pg8000/commit/3a2e7439ae3613367ec231218d7e0f541466d1e5#commitcomment-43174891 is resolved
 gunicorn==20.0.4