I suppose this is to avoid a compiler warning more than a real problem?
Trent, could you explain?

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Fix warnings on 64-bit build build of signalmodule.c

- Though I know that SIG_DFL and SIG_IGN are just small constants, there are
  cast to function pointers so the appropriate Python call is
  PyLong_FromVoidPtr so that the pointer value cannot overflow on Win64 where
  sizeof(long) < sizeof(void*).

As I understand signal handling, the signal() function takes a callback and returns a callback -- what they do is beside the point here. A callback can either be a function pointer or one of some special #define'd flags (SIG_DFL [default], SIG_IGN [ignore], others?). Practically speaking, these constants are likely to be 0 and 1 or, in any case, *not* big enough to overflow a 32-bit value. Theoretically, on Win64 there is room for overflow if these constants are treated as a long because, I think, they are only guaranteed to fit in a pointer (at least they are cast freely to function pointers).

So, effectively you are right. This patch is mainly to avoid compiler warnings, with a little healthy anal-retentiveness to boot.