remove Field generic type var _Nullable (#58)

* remove Field generic type var _Nullable

pyarrow-stubs/__lib_pxi/types.pyi

@@ -147,7 +147,7 @@ _K = TypeVar("_K", bound=_BasicDataType)
 
 class MapType(DataType, Generic[_K, _ValueT, _Ordered]):
     @property
-    def key_field(self) -> Field[_K, Literal[False]]: ...
+    def key_field(self) -> Field[_K]: ...
     @property
     def key_type(self) -> _K: ...
     @property
@@ -245,13 +245,11 @@ def ensure_metadata(
     meta: Mapping[bytes | str, bytes | str] | KeyValueMetadata | None, allow_none: bool = False
 ) -> KeyValueMetadata | None: ...
 
-_Nullable = TypeVar("_Nullable", bound=Literal[True, False], default=Literal[True])
-
-class Field(_Weakrefable, Generic[_DataType_CoT, _Nullable]):
+class Field(_Weakrefable, Generic[_DataType_CoT]):
     def equals(self, other: Field, check_metadata: bool = False) -> bool: ...
     def __hash__(self) -> int: ...
     @property
-    def nullable(self) -> _Nullable: ...
+    def nullable(self) -> bool: ...
     @property
     def name(self) -> str: ...
     @property
@@ -260,7 +258,7 @@ class Field(_Weakrefable, Generic[_DataType_CoT, _Nullable]):
     def remove_metadata(self) -> None: ...
     def with_type(self, new_type: _DataTypeT) -> Field[_DataTypeT]: ...
     def with_name(self, name: str) -> Self: ...
-    def with_nullable(self, nullable: _Nullable) -> Field[_DataType_CoT, _Nullable]: ...
+    def with_nullable(self, nullable: bool) -> Field[_DataType_CoT]: ...
     def flatten(self) -> list[Field]: ...
     def _export_to_c(self, out_ptr: int) -> None: ...
     @classmethod
@@ -320,13 +318,8 @@ def unify_schemas(
 def field(name: SupportArrowSchema) -> Field: ...
 @overload
 def field(
-    name: str,
-    type: _DataTypeT,
-) -> Field[_DataTypeT, Literal[True]]: ...
-@overload
-def field(
-    name: str, type: _DataTypeT, nullable: _Nullable, metadata: dict | None = None
-) -> Field[_DataTypeT, _Nullable]: ...
+    name: str, type: _DataTypeT, nullable: bool = ..., metadata: dict | None = None
+) -> Field[_DataTypeT]: ...
 def null() -> NullType: ...
 def bool_() -> BoolType: ...
 def uint8() -> Uint8Type: ...