py, unix: Allow to compile with -Wunused-parameter.

See issue #699.

py/parsenumbase.c

@@ -29,6 +29,7 @@
 // find real radix base, and strip preceding '0x', '0o' and '0b'
 // puts base in *base, and returns number of bytes to skip the prefix
 mp_uint_t mp_parse_num_base(const char *str, mp_uint_t len, mp_uint_t *base) {
+    (void)len; // TODO use given len?
     const byte *p = (const byte*)str;
     unichar c = *(p++);
     if ((*base == 0 || *base == 16) && c == '0') {