Add comment to explain the skip

Author: vstinner

Lib/test/test_capi/test_float.py

@@ -187,6 +187,9 @@ def test_pack_unpack_roundtrip_for_nans(self):
         if (not arch_32bit) or (sys.platform == 'win32'):
             sizes = (2, 4, 8)
         else:
+            # Skip 16-bit float and 32-bit float tests on x86 (32-bit), since
+            # these tests fail. The problem is that sNaN become qNaN just by
+            # the calling convention, there is no way to preserve sNaN.
             sizes = (8,)
 
         for _ in range(1000):