[Cython AIO] Fix Aio tests time out issue (#37917)

### What's happening
Some of our asyncio tests began timing out following a Cython upgrade to 3.0. This issue occurs consistently across both our Bazel and setup.py test environments.

### Why the time out
After some investigation, we found that our code here:4ffcdd4ab7/src/python/grpcio/grpc/_cython/_cygrpc/aio/completion_queue.pyx.pxi (L115-L116)

Was translated to this in Cython 0.29:
```
__pyx_f_7_cython_6cygrpc__unified_socket_write(__pyx_v_self->_write_fd);
```

And it changed to this in Cython 3.0:
```
__pyx_f_7_cython_6cygrpc__unified_socket_write(__pyx_v_self->_write_fd); if (unlikely(__Pyx_ErrOccurredWithGIL())) __PYX_ERR(7, 136, __pyx_L1_error)
```

Which indicates that this `nogil` function `_unified_socket_write` now requires GIL.

### What's new in Cython 3
* Cython 3 `cdef` functions with `void` return type will default to use `except *` as exception specification.
* If function have `void` return type and defined as `nogil`, Cython will always re-acquire the GIL after the function call to check if an exception has been raised.
* In some cases, this will cause a deadlock, especially if the function was called inside another `nogil` function.

### What's the fix
* This PR changes those functions to use `noexcept` as exception specification since we don't expect them to throw any exception, and this is also the suggested workarounds in Cython documentation: https://cython.readthedocs.io/en/latest/src/userguide/language_basics.html#error-return-values

### Test
* Tested locally by running `bazel test`, time our rate decreased from 5% to 0.3% in 3000 runs and 10s test time out.
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Closes #37917

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/37917 from XuanWang-Amos:fix_cython_aio 063d27aee9
PiperOrigin-RevId: 685851320

src/python/grpcio/grpc/_cython/_cygrpc/aio/completion_queue.pxd.pxi

@@ -23,7 +23,7 @@ IF UNAME_SYSNAME == "Windows":
         int win_socket_send "send" (WIN_SOCKET s, const char *buf, int len, int flags)
 
 
-cdef void _unified_socket_write(int fd) nogil
+cdef void _unified_socket_write(int fd) noexcept nogil
 
 
 cdef class BaseCompletionQueue:
@@ -48,5 +48,5 @@ cdef class PollerCompletionQueue(BaseCompletionQueue):
     cdef object _write_socket   # socket.socket
     cdef dict _loops            # Mapping[asyncio.AbstractLoop, _BoundEventLoop]
 
-    cdef void _poll(self) nogil
+    cdef void _poll(self) noexcept nogil
     cdef shutdown(self)

src/python/grpcio/grpc/_cython/_cygrpc/aio/completion_queue.pyx.pxi

@@ -22,12 +22,12 @@ cdef float _POLL_AWAKE_INTERVAL_S = 0.2
 cdef bint _has_fd_monitoring = True
 
 IF UNAME_SYSNAME == "Windows":
-    cdef void _unified_socket_write(int fd) nogil:
+    cdef void _unified_socket_write(int fd) noexcept nogil:
         win_socket_send(<WIN_SOCKET>fd, b"1", 1, 0)
 ELSE:
     from posix cimport unistd
 
-    cdef void _unified_socket_write(int fd) nogil:
+    cdef void _unified_socket_write(int fd) noexcept nogil:
         unistd.write(fd, b"1", 1)
 
 
@@ -94,7 +94,7 @@ cdef class PollerCompletionQueue(BaseCompletionQueue):
         else:
             self._loops[loop] = _BoundEventLoop(loop, self._read_socket, self._handle_events)
 
-    cdef void _poll(self) nogil:
+    cdef void _poll(self) noexcept nogil:
         cdef grpc_event event
         cdef CallbackContext *context