Fix Kokoro publish-docs job

[busunkim96]

Library docs are available here: https://googleapis.dev/python/python-ndb/latest

[cguardia]

@busunkim96 this looks good to go, sorry that we took so long to get to it. You will need to update the branch before merging. Thanks!

[busunkim96]

@cguardia Thank you for bearing with me as I tweaked this :)

[busunkim96]

@andrewsg @cguardia

Two questions:

1. It looks like a system test is failing. Should I try re-running Kokoro, or is this a known flaky test?
2. I see a pypy unit test run that's getting skipped because pypy isn't installed in the Docker image. Should I add it to the Python image?

[cguardia]

@busunkim96 hi. For 1), yes, we have some tests that depend on eventual consistency which fail every now and then, so rerunning the tests might fix it. For 2), I would defer to @andrewsg, but I believe if we have it in the configuration, it would be good to have it in the image.