py/compile: Don't await __aiter__ special method in async-for.

MicroPython's original implementation of __aiter__ was correct for an
earlier (provisional) version of PEP492 (CPython 3.5), where __aiter__ was
an async-def function.  But that changed in the final version of PEP492 (in
CPython 3.5.2) where the function was changed to a normal one.  See
https://www.python.org/dev/peps/pep-0492/#why-aiter-does-not-return-an-awaitable
See also the note at the end of this subsection in the docs:
https://docs.python.org/3.5/reference/datamodel.html#asynchronous-iterators
And for completeness the BPO: https://bugs.python.org/issue27243

To be consistent with the Python spec as it stands today (and now that
PEP492 is final) this commit changes MicroPython's behaviour to match
CPython:  __aiter__ should return an async-iterable object, but is not
itself awaitable.

The relevant tests are updated to match.

See #6267.

py/compile.c

@@ -1798,7 +1798,8 @@ STATIC void compile_async_for_stmt(compiler_t *comp, mp_parse_node_struct_t *pns
     uint try_finally_label = comp_next_label(comp);
 
     compile_node(comp, pns->nodes[1]); // iterator
-    compile_await_object_method(comp, MP_QSTR___aiter__);
+    EMIT_ARG(load_method, MP_QSTR___aiter__, false);
+    EMIT_ARG(call_method, 0, 0, 0);
     compile_store_id(comp, context);
 
     START_BREAK_CONTINUE_BLOCK