The recommended path for upgrading a Python 2 AppEngine app to the new runtime is to switch datastore code to Google Cloud NDB. However, I can't find anything about unit testing code that uses Cloud NDB (which was previously possible using the Python 2 testbed library).
How can Cloud NDB code be unit tested? (apart from standard abstracting the Cloud NDB dependency yourself)
Apparently, there is no way to unit test Cloud NDB code anymore. The only testing solution available is the Cloud Datastore Emulator.