Resolve objects through the metaclass and ban manager access on instances

[UnknownPlatypus]

I have made things!

This fixes two issues around Model.objects:

• MyModel().objects was allowed while it's a runtime error -> I've used explicit __get__(self, instance: None) to solve that
• explicit objects = BookManager() are not overriding our stub-defined type for pyright/pyrefly so objects was a Manager[Book] and incorrect. -> I think I found a way to have our objects annotation to act as a true fallback using __getattr__ on the ModelBase metaclass. By nature getattr is always checked last so now it's correct for every type checker!

I had to workaround two mypy issues that made this a bit more difficult but in the end it seem to work:

• Support using Literal on __getattr__ and __getattribute__ python/mypy#8203
• Descriptor protocol is executed for properties returning descriptors python/mypy#13608

Related issues

From #3559

This also revealed that our typing of Model.objects as ClassVar[Manager[Self]] is already a bit problematic with pyright and pyrefly (see the type-ignore's). Given this exemple:

class Book(models.Model):
    objects = BookManager()
    foo = BookManager()

I get these revealed types

┌─────────────┬───────────────┬─────────────┐
│             │ Book.objects  │  Book.foo   │
├─────────────┼───────────────┼─────────────┤
│ mypy+plugin │ BookManager   │ BookManager │
├─────────────┼───────────────┼─────────────┤
│ ty          │ BookManager   │ BookManager │
├─────────────┼───────────────┼─────────────┤
│ pyright     │ Manager[Book] │ BookManager │
├─────────────┼───────────────┼─────────────┤
│ pyrefly     │ Manager[Book] │ BookManager │
└─────────────┴───────────────┴─────────────┘

To me our declaration in the stubs should not impact regular class level assignment inference and I would expect objects to have the same type as foo. The plugin should only apply the "does not exist on abstract model" bit, the rest should be regular inference.

Fixes #174
Fixes #1414
Also improves #579

AI Policy

• I have read and agree to the AI Policy, removed any "Co-Authored-By" lines attributing coding agents, and manually reviewed the final result

[github-actions]

Diff from mypy_primer, showing the effect of this PR on type check results on a corpus of open source code:

zulip (https://github.com/zulip/zulip)
+ zerver/models/users.py:696: error: Need type annotation for "objects"  [var-annotated]
- zerver/models/users.py:1165: error: Argument 1 to "list" has incompatible type "QuerySet[UserProfile, dict[str, Any]]"; expected "Iterable[RawUserDict]"  [arg-type]
+ zerver/models/users.py:1165: error: Argument 1 to "list" has incompatible type "QuerySet[Any, dict[str, Any]]"; expected "Iterable[RawUserDict]"  [arg-type]
- zerver/models/users.py:1184: error: Argument 1 to "list" has incompatible type "QuerySet[UserProfile, dict[str, Any]]"; expected "Iterable[RawUserDict]"  [arg-type]
+ zerver/models/users.py:1184: error: Argument 1 to "list" has incompatible type "QuerySet[Any, dict[str, Any]]"; expected "Iterable[RawUserDict]"  [arg-type]
- zerver/models/users.py:1194: error: Argument 1 to "list" has incompatible type "QuerySet[UserProfile, dict[str, Any]]"; expected "Iterable[RawUserDict]"  [arg-type]
+ zerver/models/users.py:1194: error: Argument 1 to "list" has incompatible type "QuerySet[Any, dict[str, Any]]"; expected "Iterable[RawUserDict]"  [arg-type]
- zerver/lib/display_recipient.py:67: error: Argument 1 to "list" has incompatible type "QuerySet[UserProfile, dict[str, Any]]"; expected "Iterable[UserDisplayRecipient]"  [arg-type]
+ zerver/lib/display_recipient.py:67: error: Argument 1 to "list" has incompatible type "QuerySet[Any, dict[str, Any]]"; expected "Iterable[UserDisplayRecipient]"  [arg-type]
- zerver/lib/display_recipient.py:83: error: Argument 1 to "list" has incompatible type "QuerySet[UserProfile, dict[str, Any]]"; expected "Iterable[UserDisplayRecipient]"  [arg-type]
+ zerver/lib/display_recipient.py:83: error: Argument 1 to "list" has incompatible type "QuerySet[Any, dict[str, Any]]"; expected "Iterable[UserDisplayRecipient]"  [arg-type]
- zerver/actions/users.py:918: error: Incompatible types in assignment (expression has type "QuerySet[UserProfile, dict[str, Any]]", variable has type "list[dict[str, Any]]")  [assignment]
+ zerver/actions/users.py:918: error: Incompatible types in assignment (expression has type "QuerySet[Any, dict[str, Any]]", variable has type "list[dict[str, Any]]")  [assignment]
- zerver/tests/test_users.py:1846: error: "UserProfile" has no attribute "realm_id"; maybe "realm"?  [attr-defined]
- zerver/tests/test_bots.py:639: error: "UserProfile" has no attribute "bot_owner_id"  [attr-defined]
- zerver/tests/test_bots.py:2212: error: "UserProfile" has no attribute "bot_owner_id"  [attr-defined]

[sobolevn]

Looks solid! Thanks a lot, this is much better now!

[UnknownPlatypus]

Should we apply a similar pattern to our defined manager ? For ex on the User model we still use classvar

[sobolevn]

We should unify all type-checker results as much as possible :)