[3.7] Fix compiler warning in call_readline() (GH-10820)

[vstinner]

Replace strncpy() with memcpy() in call_readline() to fix the
following warning, the NUL byte is written manually just after:

Modules/readline.c: In function ‘call_readline’:
Modules/readline.c:1303:9: warning: ‘strncpy’ output truncated before
terminating nul copying as many bytes from a string as its length
[-Wstringop-truncation]
strncpy(p, q, n);
^~~~~~~~~~~~~~~~
Modules/readline.c:1279:9: note: length computed here
n = strlen(p);
^~~~~~~~~

(cherry picked from commit 1600f60)