fix(serializers): BaseSerializer should return Self (#39)

This fixes invalid type in following code:

```python
    def factory(
        serializer: Type[ModelSerializer],
        data: Dict[str, str],
    ) -> ModelSerializer:
        return serializer(data=data)
```

Without Self returned type is "BaseSerializer".

pyproject.toml

@@ -17,6 +17,7 @@ packages = [
 
 [tool.poetry.dependencies]
 python = "^3.7"
+typing_extensions = { version = "^4.6", python = "<3.11" }
 
 [tool.poetry.dev-dependencies]
 ipython = "^7.19"

rest_framework-stubs/serializers.pyi

@@ -11,6 +11,7 @@ from typing import (
     Literal,
     NoReturn,
 )
+from typing_extensions import Self
 
 from django.db import models
 from django.db.models import Manager, QuerySet
@@ -82,7 +83,7 @@ class BaseSerializer(Field[Any, Any, Any, Any]):
     instance: Any | None
     initial_data: Any
     _context: dict[str, Any]
-    def __new__(cls, *args: Any, **kwargs: Any) -> BaseSerializer: ...
+    def __new__(cls, *args: Any, **kwargs: Any) -> Self: ...
     def __class_getitem__(cls, *args: Any, **kwargs: Any) -> Any: ...
     def __init__(
         self,