PrivateAttr causes failing type check

[aphedges]

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())":

        pydantic version: 1.7
       pydantic compiled: True
            install path: /Users/ahedges/.pyenv/versions/3.7.8/envs/sdf/lib/python3.7/site-packages/pydantic
          python version: 3.7.8 (default, Sep 16 2020, 18:33:23)  [Clang 11.0.3 (clang-1103.0.32.59)]
                platform: Darwin-19.6.0-x86_64-i386-64bit
optional deps. installed: ['typing-extensions']

I created a file with the following code:

from pydantic import BaseModel, Field, PrivateAttr

class TestObject(BaseModel):
    public_field: str = Field()
    _private_field: str = PrivateAttr()

I then ran mypy (version 0.790) using the pydantic plugin. I received the following type error:

test.py:5: error: Incompatible types in assignment (expression has type "PrivateAttr", variable has type "str")
Found 1 error in 1 file (checked 1 source file)

PrivateAttr should likely not cause a type error here, similarly to how Field works.

[samuelcolvin]

PR welcome to fix this. Basically PrivateAttr need to be a function which returns Any