subprocess: Use Cython DEFs for constants

Author: 1st1

uvloop/handles/process.pyx

@@ -306,10 +306,10 @@ cdef class UVProcess(UVHandle):
         self._close()
 
 
-_CALL_PIPE_DATA_RECEIVED = 0
-_CALL_PIPE_CONNECTION_LOST = 1
-_CALL_PROCESS_EXITED = 2
-_CALL_CONNECTION_LOST = 3
+DEF _CALL_PIPE_DATA_RECEIVED = 0
+DEF _CALL_PIPE_CONNECTION_LOST = 1
+DEF _CALL_PROCESS_EXITED = 2
+DEF _CALL_CONNECTION_LOST = 3
 
 
 @cython.no_gc_clear