win32: fix error code checking in _cancel_overlapped_io

it may also report ERROR_IO_INCOMPLETE when io is in progress
diff --git a/serial/win32.py b/serial/win32.py
index cef5568..2fddf6b 100644
--- a/serial/win32.py
+++ b/serial/win32.py
@@ -219,6 +219,7 @@
 MAXDWORD = 4294967295  # Variable c_uint
 EV_RLSD = 32  # Variable c_int
 ERROR_SUCCESS = 0
+ERROR_IO_INCOMPLETE = 996
 ERROR_IO_PENDING = 997  # Variable c_long
 MS_CTS_ON = 16  # Variable c_ulong
 EV_EVENT1 = 2048  # Variable c_int