Can't assign entry to `__annotations__` unless a class has existing annotation

Bug report

Can't assign entry to __annotations__ unless a class has existing annotation. This is related to https://github.com/python/cpython/issues/88067.

python 3.9:

Python 3.9.7 | packaged by conda-forge | (default, Sep 29 2021, 20:33:18)
[Clang 11.1.0 ] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> class Foo:
...     __annotations__["b"] = int
...
>>> Foo.__annotations__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: type object 'Foo' has no attribute '__annotations__'
>>> class Foo:
...     a: int
...     __annotations__["b"] = int
...
>>> Foo.__annotations__
{'a': <class 'int'>, 'b': <class 'int'>}

python 3.10/python 3.11-rc:

Python 3.10.5 (main, Jul 12 2022, 11:32:11) [GCC 10.2.1 20210110] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> class Foo:
...     __annotations__["a"] = int
...
>>> Foo.__annotations__
{}
>>> class Foo:
...     a: float
...     __annotations__["b"] = int
...
>>> Foo.__annotations__
{'a': <class 'float'>, 'b': <class 'int'>}

Your environment

• CPython versions tested on: 3.9, 3.10, 3.11-rc
• Operating system and architecture: Mac, Linux

I'm interested in submitting a PR for this, but first would like to double check that such PR/change would be acceptable?

Beginne damit, die Beispiele im Klassenrumpf unter den betroffenen Python-Versionen zu reproduzieren, und lies den zugehörigen Issue 88067, um das umgebende Verhalten von Annotationen zu verstehen. Verfolge, wie annotations auf Klassenebene initialisiert wird, und überprüfe, dass das Zuweisen eines Eintrags in einer Klasse ohne vorherige Annotationen diesen Eintrag beibehält, ohne das bestehende Verhalten annotierter Klassen zu ändern.