The documentation for the SIG* constants also mentions:

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SIG*
All the signal numbers are defined symbolically. For example, the hangup signal is defined as signal.SIGHUP; the variable names are identical to the names used in C programs, as found in <signal.h>. The Unix man page for ‘signal()’ lists the existing signals (on some systems this is signal(2), on others the list is in signal(7)). Note that not all systems define the same set of signal names; only those names defined by the system are defined by this module.
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This is however mentioned halfway through the list of constants.  I'm not sure why this is so, IMHO it would be clearer to move the fragment I quote to the start of the module constants section.

We shouldn't define SIGCLD on platforms that don't have this name available in their libc headers, which includes macOS.  If the fragment above were moved to the start of the section no further updates would be required (IMHO).