TensorFlow is just the latest library that is Py2 only.
App Engine is Py2 only. 
The bigger issue is the 2 biggest weakness with Py2 that were not addressed in Py3 (CPython version at least)  - GIL &amp; JIT. JIT is addressed more or less by PyPy but it is Py2.7-compatible, not Py3 compatible. Addressing this in the standard CPython implementation can make for a very compelling case for move to Py3. Otherwise, it does not buy you much.