VoidPtr: Adapt `repr` and `str` methods to properly handle strings in Python 2 and 3

>>> from PySide2.QtCore import *
>>> from PySide2.support import VoidPtr
>>> o = QObject()
>>> p = VoidPtr(o)
>>> print(p)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __str__ returned non-string (type bytes)
>>> print(str(p))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __str__ returned non-string (type bytes)
>>> print(repr(p))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __repr__ returned non-string (type bytes)
>>> print(int(p))
94198306228352