Lazy import fails in some files

[rws]

When I move the function create_RealField from rings/real_mpfr.pyx to the file rings/real_field.py and put these lines into arith/misc.py:

from sage.misc.lazy_import import lazy_import
lazy_import('sage.rings.real_field', 'create_RealField', deprecation=24511)

then trying to import will give me the expected message:

sage: from sage.arith.misc import create_RealField
sage: create_RealField()
/home/ralf/sage/local/lib/python2.7/site-packages/IPython/core/interactiveshell.py:2882: DeprecationWarning: 
Importing create_RealField from here is deprecated. If you need to use it, please import it directly from sage.rings.real_field
See http://trac.sagemath.org/24511 for details.
  exec(code_obj, self.user_global_ns, self.user_ns)
Real Field with 53 bits of precision

But when I put it in rings/real_mpfr.pyx trying to import gets me:

sage: from sage.rings.real_mpfr import create_RealField
---------------------------------------------------------------------------
ImportError                               Traceback (most recent call last)
<ipython-input-1-8a8c7bec5f0f> in <module>()
----> 1 from sage.rings.real_mpfr import create_RealField

ImportError: cannot import name create_RealField

Importing from real_field directly works nicely. So, I'm conjecturing that lazy_import does not work in Cython files.

[jdemeyer]

Duplicate of #22752