py/compile: Disallow 'import *' outside module level.

This check follows CPython's behaviour, because 'import *' always populates
the globals with the imported names, not locals.

Since it's safe to do this (doesn't lead to a crash or undefined behaviour)
the check is only enabled for MICROPY_CPYTHON_COMPAT.

Fixes issue #5121.

py/compile.c

@@ -1196,6 +1196,13 @@ STATIC void compile_import_from(compiler_t *comp, mp_parse_node_struct_t *pns) {
     } while (0);
 
     if (MP_PARSE_NODE_IS_TOKEN_KIND(pns->nodes[1], MP_TOKEN_OP_STAR)) {
+        #if MICROPY_CPYTHON_COMPAT
+        if (comp->scope_cur->kind != SCOPE_MODULE) {
+            compile_syntax_error(comp, (mp_parse_node_t)pns, "import * not at module level");
+            return;
+        }
+        #endif
+
         EMIT_ARG(load_const_small_int, import_level);
 
         // build the "fromlist" tuple