move BIP85 class to __init__.py to avoid conflict with the file

[secinthenet]

Since there is both a folder/package named bip85 and a file, importing it can cause confusion and issues. For example from bip85 import app can fail because Python may try to import it from bip85/bip85.py instead of bip85/app.py, depending on sys.path, PYTHONPATH, etc.