In Python 3, imp.load_source is undocumented (apparently it was deprecated and removed via http://bugs.python.org/issue14551 with some discussion of how it is "fundamentally broken" in https://mail.python.org/pipermail/python-ideas/2014-December/030265.html).
On Windows under Python 3.3, the invoke tests fail somewhere in the compatibility implementation of imp.load_source.
An alternative approach to loading config from Python files should be implemented for Python 3. Would something like the following work?
import tokenize
values = {}
with tokenize.open(filename) as f:
exec(f.read(), values)