docs: remove line about Python 2.7 being unsupported (#808)
docs: remove line about Python 2.7 being unsupported (#808)

* docs: remove line about Python 2.7 being unsupported

According to the README and setup.py, we do support Python 2.7.

Also clarify that the utils module and its positional decorator
do still exist and does work in both Py2 and Py3

* Update migrating.rst
googleapis/python-ndb,googleapis/python-ndb