gh-86069: Do not ignore the third argument in slot_nb_inplace_power()

[serhiy-storchaka]

The default implementation of the nb_inplace_power slot no longer ignores the third argument, but passes it to the __ipow__ method if its value is not None.

• Issue: PyNumber_InPlacePower ignores o3 if o1 implements __ipow__ #86069

[skirpichev]

This looks fine from the technical side, but I'm not sure if this is a good idea.

1. Apparently, this 3-arg form isn't used somewhere for years.
2. There is no way to invoke this from pure-Python. Do we have some in-place shortcut for this stuff in some other language?
3. PyNumber_InPlacePower() still wrong: "The operation is done in-place when o1 supports it. This is the equivalent of the Python statement o1 **= o2 when o3 is Py_None, or an in-place variant of pow(o1, o2, o3) otherwise." There is no in-place variant of pow().

IMO, better to drop the module argument from __ipow__().

[serhiy-storchaka]

There is no in-place variant of pow().

Yes, this is hypothetical in-place variant of pow(). If there was some function, it would be used in the documentation.

The current behavior is inconsistent. If __ipow__ is not defined, then the third argument is not ignored, but passed to __pow__. If __ipow__ is defined, then the third argument is ignored. For extension classes it is not ignored, it is only ignored for Python classes.