Placing the imports in __init__.py would be a bad idea; __init__.py is used as the contents of your module object, so it is a public interface. Also, __init_.py is imported initially when your package is imported, while you don't actually need the imports to occur until your submodules need them.
The best approach is to put common code in an internal detail module, marked with a single initial underscore (meaning "private"), e.g. _imports.py, then in your other files write from ._imports import *.