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

[iennae]

Description

unblocks merging of #12544

[glasnt]

Tests appear to be failing because there's no pytest defined for Python 3+

https://github.com/GoogleCloudPlatform/python-docs-samples/blob/2d6db5de16f38e09a0bce9674bebf705a6c2a364/appengine/standard/iap/requirements-test.txt#L2

(this is a similar issue to what I am getting in another PR, will update with suggested resolution)

(why were these tests green earlier? they appear to be kokoro skipped last time this sample was run)

[iennae]

thank you @glasnt ! I was wondering what was up

[glasnt]

This will be the sort of thing you'll need to update to:

https://github.com/GoogleCloudPlatform/python-docs-samples/pull/12561/files#diff-72885259355e2caa839d2ffcc71fad928d4af9bc5dae781a408a468f0068d885R3

[davidcavazos]

I reproduced this locally. I think six is not an indirect dependency anymore. I think we can simply add it to the dependencies.