Cast datetime and timedelta to signed 64-bit int

The `NaT` type is represented as `-0`. As a result, casting to an
unsigned integral fails and throws an error. However casting to a signed
integral type does not have this problem and proceeds without issues.

Author: jakirkham

numcodecs/compat.py

@@ -102,5 +102,5 @@ def ensure_text(l, encoding='utf-8'):
 
 def handle_datetime(buf):
     if hasattr(buf, 'dtype') and buf.dtype.kind in 'Mm':
-        return buf.view('u8')
+        return buf.view('i8')
     return buf

numcodecs/compat_ext.pyx

@@ -21,7 +21,7 @@ cdef class Buffer:
         self.released = False
         if hasattr(obj, 'dtype'):
             if obj.dtype.kind in 'Mm':
-                obj = obj.view('u8')
+                obj = obj.view('i8')
             elif obj.dtype.kind == 'O':
                 raise ValueError('cannot obtain buffer from object array')
         if PY2 and isinstance(obj, array.array):