Fix compatibility with Python 3.7

[felixonmars]

Fixes many test failures like below:

======================================================================
ERROR: test_iterator (tests.models.TestModelAPIs)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/build/python-peewee/src/peewee-3.5.2/peewee.py", line 3613, in iterator
    yield self.iterate(False)
  File "/build/python-peewee/src/peewee-3.5.2/peewee.py", line 3597, in iterate
    raise StopIteration
StopIteration

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/build/python-peewee/src/peewee-3.5.2/tests/base.py", line 247, in inner
    method(self)
  File "/build/python-peewee/src/peewee-3.5.2/tests/models.py", line 989, in test_iterator
    self.assertEqual([u.username for u in query], users)
  File "/build/python-peewee/src/peewee-3.5.2/tests/models.py", line 989, in <listcomp>
    self.assertEqual([u.username for u in query], users)
RuntimeError: generator raised StopIteration

With the patch, all tests pass under both python 3.7 and 2.7 here.

[coleifer]

Thanks. So you can't raise StopIteration inside a generator according to this PEP... weird. I'm guessing this is some consequence from the asyncio abomination.

Currently, StopIteration raised accidentally inside a generator function will be interpreted as the end of the iteration by the loop construct driving the generator.

What if it's raised intentionally though? @gvanrossum needs to sit the fuck down, I'm glad he's taking his seat and backing off...so many terrible PEPs in the last couple versions.

[coleifer]

Merged, thanks again for taking the time to investigate and fix.