Annotate werkzeug wrap_file() and FileWrapper (#2519)

* Add FileWrapper protocol to wsgiref.types

* Annotate werkzeug's wrap_file and FileWrapper

* Remove empty line at end of file

* Fix _Readable protocols

stdlib/2and3/wsgiref/types.pyi

@@ -36,3 +36,9 @@ class ErrorStream(Protocol):
     def flush(self) -> None: ...
     def write(self, s: str) -> None: ...
     def writelines(self, seq: List[str]) -> None: ...
+
+class _Readable(Protocol):
+    def read(self, size: int = ...) -> bytes: ...
+# Optional file wrapper in wsgi.file_wrapper
+class FileWrapper(Protocol):
+    def __call__(self, file: _Readable, block_size: int = ...) -> Iterable[bytes]: ...