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

tests/basics/async_for.py

@@ -6,7 +6,7 @@ class AsyncIteratorWrapper:
         print('init')
         self._it = iter(obj)
 
-    async def __aiter__(self):
+    def __aiter__(self):
         print('aiter')
         return self
 

tests/basics/async_for2.py

-# test waiting within "async for" aiter/anext functions
+# test waiting within "async for" __anext__ function
 
 import sys
 if sys.implementation.name == 'micropython':
@@ -21,9 +21,8 @@ class ARange:
         self.cur = 0
         self.high = high
 
-    async def __aiter__(self):
+    def __aiter__(self):
         print('aiter')
-        print('f returned:', await f(10))
         return self
 
     async def __anext__(self):

tests/basics/async_for2.py.exp

 init
 aiter
-f start: 10
-coro yielded: 11
-coro yielded: 12
-f returned: 13
 anext
 f start: 20
 coro yielded: 21