Enable defining custom __new__ (#3265)

* Enable defining custom __new__

* See if xfail needed

* Qualify auto self

* Unconditionally defining PYBIND11_DISABLE_NEW_STYLE_INIT_WARNING. Returning pointer from "__init__" instead of reference.

* Use new style __init__

* Simplify __new__ creation

* Reviewer suggestions

* Match indentation

Co-authored-by: Ralf W. Grosse-Kunstleve <rwgk@google.com>