Fix RelatedManager not callable

[UnknownPlatypus]

I have made things!

mypy raise an exception when we try to call a RelatedManager. However, django allows that.

Here is a minimal reproducible exemple:

from django.db import models 
from django.contrib.auth.models import User

class CustomUser(User):
    staffs = models.Manager()
    
class SalesMan(models.Model):
    client = models.ManyToManyField(CustomUser)


sales_man = SalesMan.objects.first()

reveal_type(sales_man.client) # Revealed type is "django.db.models.manager.RelatedManager[test.t.CustomUser]"
sales_man.agent(manager="staffs") # error: "RelatedManager[CustomUser]" not callable

I added the missing stubs and a test validating it was failing before and passing after this patch.

Related issues

None

[sobolevn]

Thank you! 👍