Fix `__str__` call for all Python objects

[tooruu]

https://github.com/bruderstein/PythonScript/blob/52f7d7b3745ebd629ff4549365b13aac4c813c95/PythonScript/src/ScintillaWrapper.cpp#L61 Naively calling __str__() on an object assumes it is bound, which is not the case for non-instantiated classes. Would it be possible to use Python's str builtin?