>>> foo = 'oops'
>>> from . import foo as fubar   # should raise ImportError
>>> fubar
'oops'

After further investigation, the problem is that builtins.__import__ (the c port version) does not replicate the behaviour of importlib.__import__ (the python reference version):

>>> import builtins, importlib
>>> __package__ is None
True
>>> importlib.__import__('', globals(), locals(), ('foo',), 1)
ImportError
>>> builtins.__import__('', globals(), locals(), ('foo',), 1)
<module '__main__' (built-in)>

A further discrepancy is that for deeper relative imports, builtins.__import__ raises a ValueError instead of ImportError (contrary to expectation/spec):

>>> from ...... import foo
ValueError

A simple work around uses the python implementation to restore expected behaviour:

>>> builtins.__import__ = importlib.__import__
>>> from ...... import foo
ImportError
>>> from curses import ascii
>>> from . import ascii
ImportError

PS: Brett Cannon, to replicate please copy and paste lines in correct order :-)