Rearrange declarations to avoid circular imports.

Circular imports make import pxd's order sensitive, and violating this
order results in hard to understand errors.  As far as I can tell, this
rearrangement removes the circular nature, and has no other side
affects.

Signed-off-by: Keith Rothman <537074+litghost@users.noreply.github.com>

capnp/helpers/non_circular.pxd

@@ -1,5 +1,4 @@
 from cpython.ref cimport PyObject
-from capnp.includes.capnp_cpp cimport AsyncIoStream, WaitScope
 from libcpp cimport bool
 
 cdef extern from "capnp/helpers/capabilityHelper.h":
@@ -18,10 +17,3 @@ cdef extern from "capnp/helpers/rpcHelper.h":
 cdef extern from "capnp/helpers/asyncHelper.h":
     cdef cppclass PyEventPort:
         PyEventPort(PyObject *)
-
-cdef extern from "capnp/helpers/asyncIoHelper.h":
-    cdef cppclass AsyncIoStreamReadHelper:
-        AsyncIoStreamReadHelper(AsyncIoStream *, WaitScope *, size_t)
-        bool poll()
-        size_t read_size()
-        void* read_buffer()