typing: correct pytype mistakes in `mercurial/vfs.py` With the previous changes in this series (prior to merging the *.pyi file), this wasn't too bad- the only definitively wrong things were the `data` argument to `writelines()`, and the return type on `backgroundclosing()` (both of these errors were dropped in the previous commit; for some reason pytype doesn't like `contextlib._GeneratorContextManager`, even though that's what it determined it is): File "/mnt/c/Users/Matt/hg/mercurial/vfs.py", line 411, in abstractvfs: Bad return type 'contextlib._GeneratorContextManager' for generator function abstractvfs.backgroundclosing [bad-yield-annotation] Expected Generator, Iterable or Iterator PyCharm thinks this is `Generator[backgroundfilecloser], Any, None]`, which can be reduced to `Iterator[backgroundfilecloser]`, but pytype flagged the line that calls `yield` without an argument unless it's also `Optional`. PyCharm is happy either way. For some reason, `Iterable` didn't work for pytype: File "/mnt/c/Users/Matt/hg/mercurial/vfs.py", line 390, in abstractvfs: Function contextlib.contextmanager was called with the wrong arguments [wrong-arg-types] Expected: (func: Callable[[Any], Iterator]) Actually passed: (func: Callable[[Any, Any, Any], Iterable[Optional[Any]]]) Attributes of protocol Iterator[_T_co] are not implemented on Iterable[Optional[Any]]: __next__