Update unit tests to make flake8 happy

Author: jesse-sony

tests/test_exceptions.cpp

@@ -88,7 +88,7 @@ TEST_SUBMODULE(exceptions, m) {
     .def(py::init< std::string, int>())
     .def("getErrorCode", &BoundException::errorCode)
     .def("getMessage", &BoundException::what)
-    .def_property_readonly("message", &BoundException::what);
+    .def("__str__", &BoundException::what);
 
     m.def("throw_std_exception", []() {
         throw std::runtime_error("This exception was intentionally thrown.");

tests/test_exceptions.py

@@ -156,11 +156,11 @@ def test_bound_exceptions():
     try:
         m.throws_bound_exception()
     except m.BoundException as ex:
-        assert ex.message == "this error is a class"
+        assert str(ex) == "this error is a class"
         assert ex.getErrorCode() == 42
 
     try:
         raise m.BoundException("raising from python", 14)
     except m.BoundException as ex:
-        assert ex.message == "raising from python"
+        assert str(ex) == "raising from python"
         assert ex.getErrorCode() == 14