fix(gae): resolve linting issues in main_test.py (GoogleCloudPlatform#12557)

* fix(gae): resolve linting issues in main_test.py

* 🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

* fix: resolve linting issues where pytest not available for python versions

* add requirement six

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Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>
Co-authored-by: David Cavazos <dcavazos@google.com>

Author: 3 people

appengine/standard/iap/requirements.txt

@@ -2,3 +2,4 @@ Flask==1.1.4; python_version < '3.0'
 Flask==2.1.0; python_version > '3.0'
 Werkzeug==1.0.1; python_version < '3.0'
 Werkzeug==3.0.3; python_version > '3.0'
+six==1.16.0