Add missing attributes for docutils.io

`error_string` and `ErrorOutput` were added in `v0.19`.
Fills out some missing pieces of `Input` and `FileInput` as well.

stubs/docutils/docutils/io.pyi

@@ -1,5 +1,5 @@
 from _typeshed import OpenBinaryModeReading, OpenBinaryModeWriting, OpenTextModeReading, OpenTextModeWriting
-from typing import Any, ClassVar
+from typing import Any, AnyStr, ClassVar, Pattern
 
 from docutils import TransformSpec
 
@@ -9,11 +9,17 @@ class InputError(IOError): ...
 class OutputError(IOError): ...
 
 def check_encoding(stream: Any, encoding: str) -> bool | None: ...
+def error_string(err: str) -> str: ...
 
 class Input(TransformSpec):
     component_type: ClassVar[str]
     default_source_path: ClassVar[str | None]
     def read(self) -> Any: ...
+    def decode(self, data: AnyStr) -> str: ...
+    coding_slug: ClassVar[Pattern[bytes]]
+    byte_order_marks: ClassVar[tuple[tuple[bytes, str], ...]]
+    def determine_encoding_from_data(self, data: AnyStr) -> str: ...
+    def isatty(self) -> bool: ...
     def __getattr__(self, name: str) -> Any: ...  # incomplete
 
 class Output(TransformSpec):
@@ -29,6 +35,18 @@ class Output(TransformSpec):
     def write(self, data: str) -> Any: ...  # returns bytes or str
     def encode(self, data: str) -> Any: ...  # returns bytes or str
 
+class ErrorOutput:
+    def __init__(
+        self,
+        destination: Any | None = ...,
+        encoding: str | None = ...,
+        encoding_errors: str | None = ...,
+        decoding_errors: str | None = ...,
+    ) -> None: ...
+    def write(self, data: AnyStr) -> None: ...
+    def close(self) -> None: ...
+    def isatty(self) -> bool: ...
+
 class FileInput(Input):
     def __init__(
         self,
@@ -39,6 +57,7 @@ class FileInput(Input):
         autoclose: bool = ...,
         mode: OpenTextModeReading | OpenBinaryModeReading = ...,
     ) -> None: ...
+    def read(self) -> str: ...
     def readlines(self) -> list[str]: ...
     def close(self) -> None: ...