QImage modifies supposedly immutable strings

When QImage is passed a Python string it appears to be failing to create a local copy. This results in two issues: One is that setPixel modifies the supposedly immutable Python string causing all of havoc. Two is that it doesn't appear to hold a strong reference to the string so if the string becomes deallocated the application can crash.

Example of string mutation:

In [14]: app = QApplication([])

In [16]: s = '\xff\xff\xff\xff'

In [17]: img = QImage(s, 1, 1, QImage.Format_ARGB32)

In [19]: img.pixel(0, 0)
Out[19]: 4294967295L

In [20]: s
Out[20]: '\xff\xff\xff\xff'

In [22]: img.setPixel(0, 0, 4294967290)

In [23]: s
Out[23]: '\xfa\xff\xff\xff'

In [25]: img.setPixel(0, 0, 0)

In [26]: s
Out[26]: '\x00\x00\x00\x00'