Relax _ConverterType to work with chained converters

I guess we could add a separate type for chained converters but I don't think
that's worth it.

changelog.d/618.change.rst

@@ -1,2 +1,4 @@
 Added ``attr.converters.chain()``.
 The feature allows combining multiple conversion callbacks into one.
+
+As part of this feature, we had to relax the type information for converter callables.

src/attr/__init__.pyi

@@ -38,7 +38,7 @@ _T = TypeVar("_T")
 _C = TypeVar("_C", bound=type)
 
 _ValidatorType = Callable[[Any, Attribute[_T], _T], Any]
-_ConverterType = Callable[[Any], _T]
+_ConverterType = Callable[[Any], Any]
 _FilterType = Callable[[Attribute[_T], _T], bool]
 _ReprType = Callable[[Any], str]
 _ReprArgType = Union[bool, _ReprType]
@@ -76,7 +76,7 @@ class Attribute(Generic[_T]):
     order: bool
     hash: Optional[bool]
     init: bool
-    converter: Optional[_ConverterType[_T]]
+    converter: Optional[_ConverterType]
     metadata: Dict[Any, Any]
     type: Optional[Type[_T]]
     kw_only: bool
@@ -136,7 +136,7 @@ def attrib(
     init: bool = ...,
     metadata: Optional[Mapping[Any, Any]] = ...,
     type: Optional[Type[_T]] = ...,
-    converter: Optional[_ConverterType[_T]] = ...,
+    converter: Optional[_ConverterType] = ...,
     factory: Optional[Callable[[], _T]] = ...,
     kw_only: bool = ...,
     eq: Optional[bool] = ...,
@@ -155,7 +155,7 @@ def attrib(
     init: bool = ...,
     metadata: Optional[Mapping[Any, Any]] = ...,
     type: Optional[Type[_T]] = ...,
-    converter: Optional[_ConverterType[_T]] = ...,
+    converter: Optional[_ConverterType] = ...,
     factory: Optional[Callable[[], _T]] = ...,
     kw_only: bool = ...,
     eq: Optional[bool] = ...,
@@ -174,7 +174,7 @@ def attrib(
     init: bool = ...,
     metadata: Optional[Mapping[Any, Any]] = ...,
     type: object = ...,
-    converter: Optional[_ConverterType[_T]] = ...,
+    converter: Optional[_ConverterType] = ...,
     factory: Optional[Callable[[], _T]] = ...,
     kw_only: bool = ...,
     eq: Optional[bool] = ...,

src/attr/converters.pyi

@@ -3,11 +3,9 @@ from . import _ConverterType
 
 _T = TypeVar("_T")
 
-def chain(*validators: _ConverterType[_T]) -> _ConverterType[_T]: ...
-def optional(
-    converter: _ConverterType[_T],
-) -> _ConverterType[Optional[_T]]: ...
+def chain(*validators: _ConverterType) -> _ConverterType: ...
+def optional(converter: _ConverterType) -> _ConverterType: ...
 @overload
-def default_if_none(default: _T) -> _ConverterType[_T]: ...
+def default_if_none(default: _T) -> _ConverterType: ...
 @overload
-def default_if_none(*, factory: Callable[[], _T]) -> _ConverterType[_T]: ...
+def default_if_none(*, factory: Callable[[], _T]) -> _ConverterType: ...