Consolidate Python 2/3 compatibility code (#49)

* Consolidate Python compatibility code

Move all of the Python compatibility code into a single module under
`dask_image` instead of per subpackage. Should make it a bit easier to
manage.

* Use shared Python 2/3 compatibility module

Update all of the subpackage code to use the shared Python 2/3
compatibility module for their needs.

* Consolidate Python 2/3 compatibility tests

Move the Python 2/3 compatibility tests into a shared module as was done
with the compatibility code itself. Should make this test code easier to
manage as well.

* Test shared Python 2/3 compatibility module

Update Python 2/3 compatibility tests to use the shared compatibility
module for testing.

dask_image/imread/__init__.py

@@ -15,7 +15,7 @@
 import numpy
 import pims
 
-from . import _pycompat
+from .. import _pycompat
 
 
 def imread(fname, nframes=1):

dask_image/ndmeasure/__init__.py

@@ -16,7 +16,7 @@
 import dask.array
 
 from . import _compat
-from . import _pycompat
+from .. import _pycompat
 from . import _utils
 
 

dask_image/ndmeasure/_compat.py

@@ -7,7 +7,7 @@
 
 import dask.array
 
-from . import _pycompat
+from .. import _pycompat
 
 
 def _asarray(a):

dask_image/ndmeasure/_utils.py

@@ -10,7 +10,7 @@
 import dask.array
 
 from . import _compat
-from . import _pycompat
+from .. import _pycompat
 
 
 def _norm_input_labels_index(input, labels=None, index=None):

tests/test_dask_image/test_imread/test__pycompat.py → tests/test_dask_image/test__pycompat.py

@@ -4,19 +4,19 @@
 
 from __future__ import absolute_import
 
-import dask_image.imread._pycompat
+import dask_image._pycompat
 
 
 def test_irange():
-    r = dask_image.imread._pycompat.irange(5)
+    r = dask_image._pycompat.irange(5)
 
     assert not isinstance(r, list)
 
     assert list(r) == [0, 1, 2, 3, 4]
 
 
 def test_izip():
-    r = dask_image.imread._pycompat.izip([1, 2], [3, 4, 5])
+    r = dask_image._pycompat.izip([1, 2], [3, 4, 5])
 
     assert not isinstance(r, list)