Use field generic types for descriptors

[md384]

I have made things!

Related issues

Refs #336

This PR tries to address typing for descriptors on fields that use generic types, e.g.

On current master, you get the following

from django.db import models
from django.db.models import fields

class User(models.Model):
    id = models.AutoField(primary_key=True)
    my_custom_field1 = fields.Field[int, int]()
    
  user = User()
  reveal_type(user.my_custom_field1)  # N: Revealed type is "Any"

  user = User(my_custom_field1="hey")  # No error on the set type being str instead of int
  reveal_type(user.my_custom_field1)  # N: Revealed type is "Any"

with the changes in this PR

from django.db import models
from django.db.models import fields

class User(models.Model):
    id = models.AutoField(primary_key=True)
    my_custom_field1 = fields.Field[int, int]()
    
  user = User()
  reveal_type(user.my_custom_field1)  # N: Revealed type is "builtins.int"

  user = User(my_custom_field1="hey")  # error: Incompatible type for "my_custom_field1" of "User" (got "str", expected "int")  [misc]
  reveal_type(user.my_custom_field1)  # N: Revealed type is "builtins.int"

[md384]

This is incorrect on master too and does not pick up on null=True. When debugging, the instance seemed correct and the args were optional so I think this issue is coming from another piece of code somewhere.

[flaeppe]

Slightly off topic but still related:

I think #1900 is involved and/or need to be considered regarding null=True and optional.

IMO we should finish explore that approach primarily, before moving on to a plugin approach.

[Viicos]

Would really like to see #1900 making it as well, although in its current form it doesn't really simplify creating custom field subclasses (and I believe addressing #1900 (comment) would help in this case but requires significant work on mypy)

[flaeppe]

Slightly off topic but still related:

I think #1900 is involved and/or need to be considered regarding null=True and optional.

IMO we should finish explore that approach primarily, before moving on to a plugin approach.

[flaeppe]

Looks great. I just have 1 minor thought about the message

[flaeppe]

Nice, thank you!