PyPy count issue
Closing this, looks like a https://github.com/spulec/freezegun issue with PyPy.

I was upgrading a package and some of our tests on PyPy failed.
The backtrace goes down to:

```
peewee-2.4.5-py2.7.egg/peewee.py", line 2433, in _aggregate
    aggregation = fn.Count(SQL('*'))
AttributeError: 'builtin_function' object has no attribute 'Count'

```

Interesting fact, that running the same tests separately works. Only running it with the whole suite, at some point starts failing.
Any ideas what could cause this?

Thanks in advance.
