Broke circular import pkcs1.keys <-> pkcs1.primitives by moving function generate_key_pair(...) to pkcs1.keys. This makes it possible to run Python's 2to3 tool on pkcs1, and obtain code runnable on Python 3. Previously, the code thus obtained could not be run, because the generated circular imports 'from . import keys', 'from . import primitives' resulted in an ImportError due to a Python bug (http://bugs.python.org/issue13187).