Fix issue with exception being out of scope in Python 3.

src/python/grpcio/grpc/_cython/_cygrpc/aio/iomgr/resolver.pyx.pxi

@@ -35,6 +35,7 @@ cdef class _AsyncioResolver:
             res = future.result()
         except Exception as e:
             error = True
+            error_msg = str(e)
         finally:
             self._task_resolve = None
 
@@ -48,7 +49,7 @@ cdef class _AsyncioResolver:
             grpc_custom_resolve_callback(
                 <grpc_custom_resolver*>self._grpc_resolver,
                 NULL,
-                grpc_socket_error("getaddrinfo {}".format(str(e)).encode())
+                grpc_socket_error("getaddrinfo {}".format(error_msg).encode())
             )
 
     cdef void resolve(self, char* host, char* port):

src/python/grpcio/grpc/_cython/_cygrpc/aio/iomgr/socket.pyx.pxi

@@ -45,6 +45,7 @@ cdef class _AsyncioSocket:
             self._reader, self._writer = future.result()
         except Exception as e:
             error = True
+            error_msg = str(e)
         finally:
             self._task_connect = None
 
@@ -61,7 +62,7 @@ cdef class _AsyncioSocket:
         else:
             self._grpc_connect_cb(
                 <grpc_custom_socket*>self._grpc_socket,
-                grpc_socket_error("connect {}".format(str(e)).encode())
+                grpc_socket_error("connect {}".format(error_msg).encode())
             )
 
     def _read_cb(self, future):