Tracing a class that defines a custom `__class__` appears to remove said property under certain circumstances

Tested on 3.7, 3.8, 3.9 and 3.10

It appears that using super() in the class makes the class behave differently when traced:

import sys

def tracer(frame, event, _):
    return tracer
sys.settrace(tracer)


class Foo:
    pass

class FancyFoo:
    def bug(self):
        super()

    @property
    def __class__(self):
        return Foo

assert isinstance(FancyFoo(), Foo)

Remove either the def bug or the settrace call and the assertion passes.

Ref https://github.com/nedbat/coveragepy/issues/1382.

从提供的 Python 复现程序开始，重点关注 sys.settrace、super() 和自定义的 class 属性。在受影响的 Python 版本中复现失败的断言，然后跟踪相关行为，直到该断言在 tracing 期间仍然为真。回归测试应覆盖所报告的组合。