switch to relative cimports

This still does not allow to import kwant under other names:
http://thread.gmane.org/gmane.comp.python.cython.user/13809

Once we depend on Python 3.5, and "proper" relative cimports are supported by
Cython, we could add a test for exclusive use of relative imports.

kwant/_system.pyx

@@ -12,8 +12,8 @@ import numpy as np
 from scipy import sparse as sp
 from itertools import chain
 
-from kwant.graph.core cimport CGraph, gintArraySlice
-from kwant.graph.defs cimport gint
+from .graph.core cimport CGraph, gintArraySlice
+from .graph.defs cimport gint
 from .graph.defs import gint_dtype
 
 msg = ('Hopping from site {0} to site {1} does not match the '

kwant/graph/core.pxd

@@ -7,7 +7,7 @@
 # http://kwant-project.org/authors.
 
 cimport numpy as np
-from kwant.graph.defs cimport gint
+from .defs cimport gint
 
 cdef struct Edge:
     gint tail, head

kwant/graph/core.pyx

@@ -30,7 +30,7 @@ from libc.stdlib cimport malloc, realloc, free
 from libc.string cimport memset
 import numpy as np
 cimport numpy as np
-from kwant.graph.defs cimport gint
+from .defs cimport gint
 
 cdef class Graph:
     """An uncompressed graph.  Used to make compressed graphs.  (See `CGraph`.)

kwant/graph/scotch.pyx

@@ -13,9 +13,9 @@ __all__ = ['bisect', 'reset']
 cimport libc.stdio
 import numpy as np
 cimport numpy as np
-from kwant.graph cimport core
+from . cimport core
 from . import core, defs
-from kwant.graph.c_scotch cimport *
+from .c_scotch cimport *
 
 DEF SCOTCH_STRATQUALITY = 1
 DEF SCOTCH_STRATSPEED = 2

kwant/graph/slicer.pyx

@@ -10,10 +10,10 @@ import numpy as np
 cimport numpy as np
 cimport cython
 from libc.string cimport memcpy
-from kwant.graph.defs cimport gint
+from .defs cimport gint
 from .defs import gint_dtype
-from kwant.graph.core cimport CGraph
-cimport kwant.graph.c_slicer as c_slicer
+from .core cimport CGraph
+from . cimport c_slicer
 
 __all__ = ['slice']
 

kwant/graph/utils.pyx

@@ -16,9 +16,9 @@ cimport numpy as np
 from libc.stdlib cimport malloc, realloc, free
 from libc.string cimport memset
 cimport cython
-from kwant.graph.defs cimport gint
+from .defs cimport gint
 from .defs import gint_dtype
-from kwant.graph.core cimport CGraph, CGraph_malloc
+from .core cimport CGraph, CGraph_malloc
 from .core import CGraph, CGraph_malloc
 
 @cython.boundscheck(False)