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?

Start by reproducing the class-body examples on the affected Python versions and read the related issue 88067 for the surrounding annotation behavior. Trace how class-level annotations is initialized and verify that assigning an entry in a class with no prior annotations preserves that entry without changing the existing annotated-class behavior.