add (overwrite with) mypy stubs, if available

stdlib/3/codecs.pyi

@@ -0,0 +1,31 @@
+from typing import Any, BinaryIO, Callable, IO
+
+BOM_UTF8 = b''
+
+class Codec: ...
+class StreamWriter(Codec): ...
+
+class CodecInfo(tuple):
+    def __init__(self, *args) -> None: ...
+
+def register(search_function: Callable[[str], CodecInfo]) -> None:
+    ...
+
+def register_error(name: str, error_handler: Callable[[UnicodeError], Any]) -> None: ...
+
+def lookup(encoding: str) -> CodecInfo:
+    ...
+
+# TODO This Callable is actually a StreamWriter constructor
+def getwriter(encoding: str) -> Callable[[BinaryIO], StreamWriter]: ...
+
+class IncrementalDecoder:
+    errors = ...  # type: Any
+    def __init__(self, errors=''): ...
+    def decode(self, input, final=False): ...
+    def reset(self): ...
+    def getstate(self): ...
+    def setstate(self, state): ...
+
+def open(filename: str, mode: str='rb', encoding: str=None, errors: str='strict', buffering: int=1) -> IO[Any]:
+    ...