[issue43770] Rework C types initialization

[STINNER Victor]

STINNER Victor <vstinner at python.org> added the comment:

If I modify type_ready() to call type_ready_inherit() before type_ready_fill_dict(), some types create a between between their C slot (tp_init) and the Python API in tp_dict (tp_dict["__init__"]).

Example with importlib:

class FileLoader(...):
    def __init__(...):
        ...

=> FileLoader.tp_init = slot_tp_init

class SourceFileLoader(FileLoader):
    ...


When PyType_Ready() is called on SourceFileLoader, we get:

* SourceFileLoader.tp_base = FileLoader
* SourceFileLoader.tp_init = NULL
* SourceFileLoader.tp_dict has no "__init__" key

When inherit_slots() is called, SourceFileLoader.tp_init is set to slot_tp_init().

When add_operators() is called, SourceFileLoader.tp_dict["__init__"] is set to PyDescr_NewWrapper(slot_tp_init).

Problem: we a loop! tp_dict["__init__"] => slot_tp_init => tp_dict["__init__"]

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue43770>
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