remove direct import of nan

Author: Brock Mendel <jbrockmendel@gmail.com>

Closes pandas-dev#17185 from jbrockmendel/dont_import_nan and squashes the following commits:

ee260b8 [Brock Mendel] remove direct import of nan

pandas/core/sparse/frame.py

@@ -5,7 +5,6 @@
 from __future__ import division
 # pylint: disable=E1101,E1103,W0231,E0202
 
-from numpy import nan
 from pandas.compat import lmap
 from pandas import compat
 import numpy as np
@@ -156,7 +155,7 @@ def _init_dict(self, data, index, columns, dtype=None):
                 v = v.copy()
             else:
                 if isinstance(v, dict):
-                    v = [v.get(i, nan) for i in index]
+                    v = [v.get(i, np.nan) for i in index]
 
                 v = sp_maker(v)
             sdict[k] = v