Fix fletecher32's cimports

Author: jakirkham

numcodecs/fletcher32.pyx

@@ -2,13 +2,16 @@
 # cython: overflowcheck=False
 # cython: cdivision=True
 
-from numcodecs.abc import Codec
-from numcodecs.compat import ensure_contiguous_ndarray
 
 from libc.stdint cimport uint8_t, uint16_t, uint32_t
 
+from cpython.bytes cimport PyBytes_FromStringAndSize
+
 from ._utils cimport store_le32, load_le32
 
+from numcodecs.abc import Codec
+from numcodecs.compat import ensure_contiguous_ndarray
+
 
 cdef uint32_t _fletcher32(const uint8_t[::1] _data):
     # converted from