gh-143715: deprecate incomplete initialization of struct.Struct()

[skirpichev]

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

• Issue: Make struct.Struct() really immutable #143715

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• Issue: Make struct.Struct() really immutable #143715

[skirpichev]

The evil plan is to remove custom Struct.__init__() method and move all initialization logic to the Struct.__new__(). Something, that was done by #94532 before.

[serhiy-storchaka]

I think this needs a separate issue for discussion. Please open a new issue, and refer #78724 and #112358 (maybe even with citations).

We need to discuss what should be the end goal after the end of the deprecation period, and how deprecations canl help to transit the user code.

[skirpichev]

CC @meadori per experts index.

[vstinner]

LGTM.

What do you think @serhiy-storchaka?

[serhiy-storchaka]

Please add a test for a new case mentioned in the issue. We can test if the type's tp_init and tp_new are the same to make more qualified decision.

[serhiy-storchaka]

And what about the following case?

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

my_struct = MyStruct('<h')
my_struct.pack(12345)

There should emit a FutureWarning, because the current and the future code produce different results. MyStruct(5) should emit a DeprecationWarning, because it works in the current code, but will be error in future. ``MyStruct('>h')` should work without warnings.

Add *args, **kwargs to __init__() at line 798 and test with different arguments:
MyStruct(), MyStruct('>h'), MyStruct('<h'), MyStruct(5), MyStruct('<h', 5), MyStruct(format='<h'), etc.

Also add *args, **kwargs to __new__() in other example and test with different arguments.