Wrappers for non-void methods fail to compile with release_GIL_and_enable_signal = True

[krivenko]

When release_GIL_and_enable_signal is switched on, the result object is created inside a nested try block (line 547 of wrap.cxx). Later, it is assumed to be visible in the enclosing scope of that try (line 563).

cpp2py/cpp2py/mako/wrap.cxx

Lines 539 to 570 in a54c236

	try {
	${overload._get_calling_pattern1()}
	%if overload.release_GIL_and_enable_signal :
	PyOS_sighandler_t sig = PyOS_getsig(SIGINT);
	cpp2py::signal_handler::start(); // start the C++ signal handler
	Py_BEGIN_ALLOW_THREADS;
	try {
	%endif
	${overload._get_calling_pattern()}; // the call is here. It sets up "result" : sets up in the python layer.
	%if overload.release_GIL_and_enable_signal :
	}
	catch(...) {
	// an error has occurred : clean GIL, handler and rethrow
	Py_BLOCK_THREADS; // cf python include, ceval.h, line 72 comments and below.
	cpp2py::signal_handler::stop(); // stop the C++ signal handler
	PyOS_setsig(SIGINT, sig);
	throw; //
	}
	Py_END_ALLOW_THREADS;
	cpp2py::signal_handler::stop();
	PyOS_setsig(SIGINT, sig);
	%endif
	%if not overload.is_constructor :
	%if overload.rtype != "void" :
	py_result = convert_to_python(std::move(result));
	%else:
	Py_INCREF(Py_None);
	py_result = Py_None;
	%endif
	%endif
	return ${'py_result' if not py_meth.is_constructor else '0'};
	}

I wanted to prepare a minimal example based on an existing wrapping unit test, but those seem to have been disabled in both TRIQS unstable and cpp2py.