Smooth over Python 2/3 memoryview coercion

In Python 2, everything can be comfortably coerced to the old-style
buffer interface, which is easily coerced to the new buffer interface.
This is a nice path to proceed down as it can cleanly get a `memoryview`
on Python 2 without copying. This strategy works well on Python 3 as
well as long as we make `buffer` a no-op.

Author: jakirkham

numcodecs/compat.py

@@ -18,13 +18,15 @@
     binary_type = str
     integer_types = (int, long)
     reduce = reduce
+    buffer = buffer
 
 else:  # pragma: py2 no cover
 
     text_type = str
     binary_type = bytes
     integer_types = int,
     from functools import reduce
+    buffer = lambda a: a
 
 
 def buffer_tobytes(v):
@@ -33,10 +35,8 @@ def buffer_tobytes(v):
         return v
     elif isinstance(v, np.ndarray):
         return v.tobytes(order='A')
-    elif PY2:  # pragma: py3 no cover
-        v = buffer(v)
-
-    return memoryview(v).tobytes()
+    else:
+        return memoryview(buffer(v)).tobytes()
 
 
 def buffer_copy(buf, out=None):