modernize pyrex -> cython

coro/zstack_zlib.pxd

+# -*- Mode: Cython -*-
+
+cimport zlib
+
+cdef class zstack:
+    cdef zlib.z_stream squish, unsquish
+    # this buffer is only used for compression
+    cdef unsigned char * buffer
+    cdef int buffer_size
+    cdef size_t deflate (self, void * base, size_t size)
+    cdef size_t inflate (self, void * dst, size_t dsize, void * src, size_t ssize)