Hello,
In GAE Standard you cannot change the pip version, however in GAE Flex you can build your own runtime based on a GCP official image and then add a step before installing requirements to force use a specific or latest pip version using something like "python3 -m pip install --user --upgrade pip==9.0.3"
Generically speaking I recommend that you use GAE Flex for Tensorflow, but keep in mind that you need to double-check the logs for any other issues. There might be more things going wrong but it just stops at the first error.
Also, you can follow this Public Issue that seems to be almost identical to stay updated on any progress[1]