Python: fixing incorrect function object dereference.

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.

docs/changes.xml

@@ -43,6 +43,13 @@ some Spring Boot applications failed to start, notably with Grails.
 </para>
 </change>
 
+<change type="bugfix">
+<para>
+incorrect Python protocol auto detection (ASGI or WSGI) for native callable
+object, notably with Falcon.
+</para>
+</change>
+
 </changes>