The __call__ method can be native and not be a PyFunction type. A type check
is thus required before accessing op_code and other fields.
Reproduced on Ubuntu 21.04, Python 3.9.4 and Falcon framework: here, the
App.__call__ method is compiled with Cython, so accessing op_code->co_flags is
invalid; accidentally, the COROUTINE bit is set which forces the Python module
into the ASGI mode.
The workaround is explicit protocol specification.
Note: it is impossible to specify the legacy mode for ASGI.
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