I can't really figure out why, but the <util.h> import in posixmodule.c seems to get skipped since python 3.7. The condition doesn't look entirely correct in case libutil.h is also available on macOS, however this has not changed since 3.6 and it worked fine there while both where available.


Part of the configure log:

checking pty.h usability... no
checking pty.h presence... no
checking for pty.h... no
checking libutil.h usability... yes
checking libutil.h presence... yes
checking for libutil.h... yes
...
checking util.h usability... yes
checking util.h presence... yes
checking for util.h... yes


Build error:

./Modules/posixmodule.c:5924:9: error: implicit declaration of function 'openpty' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
    if (openpty(&master_fd, &slave_fd, NULL, NULL, NULL) != 0)
        ^
./Modules/posixmodule.c:5924:9: note: did you mean 'openat'?
/nix/store/q819d3vjz7vswpvkrfa9gck3ys8rmvcj-Libsystem-osx-10.11.6/include/sys/fcntl.h:480:5: note: 'openat' declared here
int     openat(int, const char *, int, ...) __DARWIN_NOCANCEL(openat) __OSX_AVAILABLE_STARTING(__MAC_10_10, __IPHONE_8_0);
        ^
./Modules/posixmodule.c:5924:9: warning: this function declaration is not a prototype [-Wstrict-prototypes]
    if (openpty(&master_fd, &slave_fd, NULL, NULL, NULL) != 0)
        ^
./Modules/posixmodule.c:6018:11: error: implicit declaration of function 'forkpty' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
    pid = forkpty(&master_fd, NULL, NULL, NULL);
          ^
./Modules/posixmodule.c:6018:11: warning: this function declaration is not a prototype [-Wstrict-prototypes]
2 warnings and 2 errors generated.
make: *** [Makefile:1793: Modules/posixmodule.o] Error 1