py/mpz: Skip separators when running out of digits to print.

This commit fixes the addition of a stray separator before the number
when printing an MPZ-backed integer and the first group is three digits
long.

This fixes #8984.

Signed-off-by: Alessandro Gatti <a.gatti@frob.it>

py/mpz.c

@@ -1717,7 +1717,7 @@ size_t mpz_as_str_inpl(const mpz_t *i, unsigned int base, const char *prefix, ch
                 break;
             }
         }
-        if (comma && (s - last_comma) == 3) {
+        if (!done && comma && (s - last_comma) == 3) {
             *s++ = comma;
             last_comma = s;
         }