Fix the following warning:

Python/pystrtod.c: In function 'format_float_short':
Python/pystrtod.c:1007:13: warning: 'strncpy' output truncated before terminating nul copying 3 bytes from a string of the same length [-Wstringop-truncation]
             strncpy(p, "ERR", 3);

(cherry picked from commit 9fb84157)