Does not honor PYTHONPATH, test case enclosed

Here's a test script proving the cycle is not detected:

```
#!/bin/bash
mkdir test-pycycle
cd test-pycycle
mkdir -p app/stuff
touch app/__init__.py
touch app/stuff/__init__.py
echo "from stuff import users" > app/things.py
echo "import things" > app/stuff/users.py
python -m virtualenv .venv
source .venv/bin/activate
pip install pycycle
PYTHONPATH=app pycycle --source app # Passes just fine
PYTHONPATH=app python app/things.py # Fails
```