[serial] Update stubtest comment (#14739)

Cf. #14718

Author: srittau

stubs/pyserial/@tests/stubtest_allowlist_win32.txt

@@ -11,6 +11,6 @@ serial.tools.list_ports_posix  # Posix only
 # names.
 serial.serialwin32.Serial.write
 
-# FIXME: new stubtest errors from mypy v1.18.1 that need to be looked at more closely.
-# See https://github.com/python/typeshed/pull/14699
+# Depending on the Windows variant, ULONG_PTR is either c_longlong or c_ulong,
+# but when running stubtest, only one of these is seen.
 serial.win32.ULONG_PTR