Fix compiler warning in structseq_repr() (GH-10841)

Replace strncpy() with memcpy() in structseq_repr() to fix the
following compiler warning:

Objects/structseq.c:187:5: warning: 'strncpy' specified bound depends on the length of the source argument [-Wstringop-overflow=]
     strncpy(pbuf, typ->tp_name, len);
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Objects/structseq.c:185:11: note: length computed here
     len = strlen(typ->tp_name) > TYPE_MAXSIZE ? TYPE_MAXSIZE :

The function writes the terminating NUL byte later.