add (overwrite with) mypy stubs, if available

stdlib/2.7/StringIO.pyi

@@ -0,0 +1,28 @@
+# Stubs for StringIO (Python 2)
+
+from typing import Any, IO, AnyStr, Iterator, Iterable, Generic
+
+class StringIO(IO[AnyStr], Generic[AnyStr]):
+    closed = ... # type: bool
+    softspace = ... # type: int
+    def __init__(self, buf: AnyStr = '') -> None: ...
+    def __iter__(self) -> Iterator[AnyStr]: ...
+    def next(self) -> AnyStr: ...
+    def close(self) -> None: ...
+    def isatty(self) -> bool: ...
+    def seek(self, pos: int, mode: int = 0) -> None: ...
+    def tell(self) -> int: ...
+    def read(self, n: int = -1) -> AnyStr: ...
+    def readline(self, length: int = None) -> AnyStr: ...
+    def readlines(self, sizehint: int = 0) -> List[AnyStr]: ...
+    def truncate(self, size: int = None) -> int: ...
+    def write(self, s: AnyStr) -> None: ...
+    def writelines(self, iterable: Iterable[AnyStr]) -> None: ...
+    def flush(self) -> None: ...
+    def getvalue(self) -> AnyStr: ...
+    def __enter__(self) -> Any: ...
+    def __exit__(self, type: Any, value: Any, traceback: Any) -> Any: ...
+    def fileno(self) -> int: ...
+    def readable(self) -> bool: ...
+    def seekable(self) -> bool: ...
+    def writable(self) -> bool: ...