[mypyc] Fix decorated __new__ in native classes

[rheard]

Fixes mypyc/mypyc#1249.

A decorated method has no entry in ClassIR.method_decls, but the function below the decorators is in ClassIR.methods. For __new__ the code generator used both, and the result didn't work for any caller:

• The constructor and tp_new called the function below the decorators directly, so a native call skipped the decorators.
• The decorators were applied to the built-in __new__ function that calls tp_new. Setting the result on the class makes Python replace tp_new, which fails the safety check of that built-in function. Every call that isn't a native constructor call raised TypeError: C.__new__(C) is not safe, use object.__new__().
• If __new__ takes other arguments than __init__, the generated C didn't compile (too many arguments to function 'CPyDef_C_____new__').
• In a class with a built-in base class, __new__ in the type dict came from the method table, whose flags don't match the wrapper function of __new__. Instantiating the class crashed.

[p-sawicki]

looks like there's an edge case where method_decls of the base class doesn't contain __new__ when it's decorated with @staticmethod and the subclass is prepared first.

when compiling the subclass it's assumed that __new__ is decorated but not when compiling the base since @staticmethod is removed so there's an inconsistency.

# multimodule test
[case testDecoratedNewInBaseClassPreparedAfterSubclass]
from other import Base

class Child(Base):
    def __init__(self) -> None:
        raise ValueError("expected init failure")

def construct() -> Child:
    return Child()

[file other.py]
from typing import Any

class Base:
    @staticmethod
    def __new__(cls) -> Any:
        return super().__new__(cls)

    def __init__(self) -> None:
        pass

[file driver.py]
from native import construct
from testutil import assertRaises

with assertRaises(ValueError):
    construct()

[rheard]

Good catch. I just pushed up a commit with a fix for this, though its worth noting this PR is going to conflict with a few of the others I've made. I'll go back and fix real conflicts once once of them gets merged in. Thanks!

[p-sawicki]

thanks!