py/warning: Support categories for warnings.

Python defines warnings as belonging to categories, where category is a
warning type (descending from exception type). This is useful, as e.g.
allows to disable warnings selectively and provide user-defined warning
types.  So, implement this in MicroPython, except that categories are
represented just with strings.  However, enough hooks are left to implement
categories differently per-port (e.g. as types), without need to patch each
and every usage.

extmod/modlwip.c

@@ -1454,7 +1454,7 @@ STATIC mp_obj_t lwip_getaddrinfo(size_t n_args, const mp_obj_t *args) {
             && (type == 0 || type == MOD_NETWORK_SOCK_STREAM)
             && proto == 0
             && flags == 0)) {
-            mp_warning("unsupported getaddrinfo constraints");
+            mp_warning(MP_WARN_CAT(RuntimeWarning), "unsupported getaddrinfo constraints");
         }
     }