Support QuerySet with Generic types

[w0rp]

I have some modified stubs for QuerySet and Manager so they accept type parameters. This makes the following possible.

for foo in Foo.objects.all(): # The type of foo is inferred as Foo

Using it requires stubbing models like so.

# foo.py
from django.db import models

class Foo(models.Model):
    bar = models.CharField(max_length=255)

# foo.pyi
from django.db import models

class Foo(models.Model):
    objects = ... # type: models.Manager['Foo']
    bar = ... # type: str

Would you be interested in adding this to the project if I create a pull request?