Add encodings.utf_8_sig stubs (#6691)

stdlib/encodings/utf_8_sig.pyi

@@ -0,0 +1,27 @@
+import codecs
+
+class IncrementalEncoder(codecs.IncrementalEncoder):
+    def __init__(self, errors: str = ...) -> None: ...
+    def encode(self, input: str, final: bool = ...) -> bytes: ...
+    def reset(self) -> None: ...
+    def getstate(self) -> int: ...  # type: ignore[override]
+    def setstate(self, state: int) -> None: ...  # type: ignore[override]
+
+class IncrementalDecoder(codecs.BufferedIncrementalDecoder):
+    def __init__(self, errors: str = ...) -> None: ...
+    def _buffer_decode(self, input: bytes, errors: str | None, final: bool) -> tuple[str, int]: ...
+    def reset(self) -> None: ...
+    def getstate(self) -> tuple[bytes, int]: ...
+    def setstate(self, state: tuple[bytes, int]) -> None: ...
+
+class StreamWriter(codecs.StreamWriter):
+    def reset(self) -> None: ...
+    def encode(self, input: str, errors: str | None = ...) -> tuple[bytes, int]: ...
+
+class StreamReader(codecs.StreamReader):
+    def reset(self) -> None: ...
+    def decode(self, input: bytes, errors: str | None = ...) -> tuple[str, int]: ...
+
+def getregentry() -> codecs.CodecInfo: ...
+def encode(input: str, errors: str | None = ...) -> tuple[bytes, int]: ...
+def decode(input: bytes, errors: str | None = ...) -> tuple[str, int]: ...