pytest-3 invocation failure due to conflicting filenames
This started happening after #350 

Repro:

$ pytest-3 -k rust -v

```
import file mismatch:
imported module 'asyncio_test' has this __file__ attribute:
  /home/arun/src/py2many/tests/cases/asyncio_test.py
which is not the same as the test file we want to collect:
  /home/arun/src/py2many/tests/expected/asyncio_test.py
HINT: remove __pycache__ / .pyc files and/or use a unique basename for your test file modules
```