* ``Struct.__new__()`` will require a mandatory argument (format)
* Calls of ``__init__()`` method on initialized Struct are deprecated

This make format argument in the __init__() - optional.  If it's
missing, the object must be already initialized in __new__().

Co-authored-by: Victor Stinner <vstinner@python.org>

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

This catch current pattern for Struct's subclassing like

class MyStruct(Struct):
    def __init__(self):
        super().__init__('>h')