datetime64 arrays don't support buffer protocol

[alimanfoo]

On attempt to encode a datetime64 array with any codec that relies on buffer access, e.g.::

  File "numcodecs/blosc.pyx", line 350, in numcodecs.blosc.Blosc.encode (numcodecs/blosc.c:4126)
  File "numcodecs/blosc.pyx", line 159, in numcodecs.blosc.compress (numcodecs/blosc.c:2274)
  File "numcodecs/compat_ext.pyx", line 29, in numcodecs.compat_ext.Buffer.__cinit__ (numcodecs/compat_ext.c:1358)
Exception: cannot include dtype 'M' in a buffer

This could be worked around in compat_ext.pyx, e.g., explicit check for datetime64 array and view as int64 before get buffer.

xref joblib/joblib#183 numpy/numpy#4983

[alimanfoo]

Resolved via #46