Cooperative multi-inheritance works with __init__ but not with __new__

[Russell Warren]

The PyQt5 cooperative multi-inheritance is implemented for __init__, but
not for __new__.  It looks like one or all of PyQt5.QtCore.QObject,
sip.wrapper, and.or sip.simplewrapper don't cooperate by invoking
super().__new__.

Can this be addressed in PyQt, or is there a fundamental reason that
calling super().__new__ is omitted?

Below is code that demonstrates the problem. You can see that __init__ is
properly invoked via super()'s MRO chain, but __new__ gets blocked by (at
least) one of the three mentioned classes. They all should invoke
super().__new__ in order to cooperate properly.

This can be worked around by being careful with ordering of bases as shown
(or probably with an adapter class), but it would be nice if PyQt fully
supported cooperative multi inheritance here.

The code:

```
from PyQt5 import QtCore

class Mixin:
    def __new__(cls, *args, **kwargs):
        print("Called Mixin.__new__")
        self = super().__new__(cls, *args, **kwargs)
        return self

    def __init__(self, age = 0, **kwargs):
        print("Called Mixin.__init__")
        super().__init__(**kwargs)

class MyQObject_bad(QtCore.QObject, Mixin): pass
class MyQObject_good(Mixin, QtCore.QObject): pass

print("Showing MyQObject_good example:")
good = MyQObject_good()

print("\nShowing MyQObject_bad example (Mixin.__new__ not called!):")
bad = MyQObject_bad()

print("\nMROs (to show that Mixin.__new__ gets blocked):")
sep = "\n - "
print(f"Good MRO:{sep}{sep.join(cls.__name__ for cls in
MyQObject_good.__mro__)}")
print(f"Bad MRO: {sep}{sep.join(cls.__name__ for cls in
MyQObject_bad.__mro__)}")
```

Output of the above script looks like this:

```
Showing MyQObject_good example:
Called Mixin.__new__
Called Mixin.__init__

Showing MyQObject_bad example (Mixin.__new__ not called!):
Called Mixin.__init__

MROs (to show that Mixin.__new__ gets blocked):
Good MRO:
 - MyQObject_good
 - Mixin
 - QObject
 - wrapper
 - simplewrapper
 - object
Bad MRO:
 - MyQObject_bad
 - QObject
 - wrapper
 - simplewrapper
 - Mixin
 - object
```
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