gh-144330: Initialize classmethod and staticmethod in new

[vstinner]

Move classmethod and staticmethod initialization from __init__() to __new__().

PyClassMethod_New() and PyStaticMethod_New() now copy attributes of the wrapped functions: __module__, __name__, __qualname__ and __doc__.

Change static type initialization: initialize PyStaticMethod_Type and PyCFunction_Type earlier.

Remove test_refleaks_in_classmethod___init__() and test_refleaks_in_staticmethod___init__() tests from test_descr since classmethod and staticmethod have no __init__() method anymore.

• Issue: classmethod and staticmethod can crash with NULL / uninitialized callables #144330

[vstinner]

cc @colesbury

[vstinner]

With this change, cm->cm_callable and sm->sm_callable cannot be NULL anymore (except in cm_dealloc() and sm_dealloc() to handle erros in cm_new() and sm_new()), but they can be equal to None: cm_new() and sm_new() don't check the callable type (no change compared to the current code).

[vstinner]

I modified my PR to keep __init__() methods for backward compatibility. For example, __init__() is needed by abc.abstractclassmethod which calls super().__init__(callable):

class abstractclassmethod(classmethod):
    __isabstractmethod__ = True
    def __init__(self, callable):
        callable.__isabstractmethod__ = True
        super().__init__(callable)

[colesbury]

LGTM

[sergey-miryanov]

LGTM