when extra is allowed, representation is break

[COCOLMAN]

Checks

• I added a descriptive title to this issue
• I have searched (google, github) for similar issues and couldn't find anything
• I have read and followed the docs and still think this is a bug

Bug

Output of python -c "import pydantic.utils; print(pydantic.utils.version_info())":

python -c "import pydantic.utils; print(pydantic.utils.version_info())"
             pydantic version: 1.8.2
            pydantic compiled: False
                 install path: /Users/Sol/Project/pydantic/pydantic
               python version: 3.9.5 (default, Jun 23 2021, 22:47:04)  [Clang 11.0.3 (clang-1103.0.32.62)]
                     platform: macOS-10.15.6-x86_64-i386-64bit
     optional deps. installed: ['devtools', 'dotenv', 'email-validator', 'typing-extensions']

from pydantic import BaseModel
from pydantic import Extra



class Foo(BaseModel):
    a: int

    class Config:
        extra = Extra.allow


f = Foo(a=1, b=2)
breakpoint()
print(f) # << raise error. not only 'print', also str(Model), breakpoint().. etc

because

field b that allowed is not in Model.fields

in main.BaseModel

class BaseModel(Representation, metaclass=ModelMetaclass):

    ...

    def __repr_args__(self) -> 'ReprArgs':
        return [(k, v) for k, v in self.__dict__.items() if self.__fields__[k].field_info.repr] # KeyError

[povilasb]

Triggered this bug as well.
Did a quick git bisect and seems like this commit is the culprit: 362f4a5