Prove tasklets can be Python 2.7 and 3.7 compatible.

[chrisrossi]

If our codebase is to be compatible with Python 2.7, we have to change
all instances where we return from a tasklet to using raise ndb.Return(), because Python 2 doesn't allow returning a value from a
generator.

Python 3.7 also doesn't allow raising StopIteration from inside of a
generator, which, on first blush, seemed to preclude having a common
codebase that was both Python 2.7 and 3.7 compatible. As it turns out,
though, we can just make tasklets.Return inherit from
Exception instead of from StopIteration.

Crisis averted.

[cguardia]

What a scare! Good thing it was fixable.

[chrisrossi]

And just to be clear: from now until we decide we can drop Python 2 support, we need to always raise Return. When we have a test suite running under 2.7, that will be enforced.

Users, however, can do whatever they want. If they're using Python 2 they'll need to raise Return, as well, but Python 3 users should feel free to just use a simple return. That will still work just fine. We just can't do it ourselves, because of backwards compatibility.