Added type hints

[radarhere]

In order to add type hints to the following Image methods

def __enter__(self):
def _close_fp(self):
def __exit__(self, *args):

I've made a few changes.

Image now has

def __enter__(self) -> Image:
def __exit__(self, *args: object) -> None:

I added an __enter__() and __exit__() to ImageFile, so that with Image.open(...): will type hint an ImageFile instance. I also moved _close_fp() to ImageFile, as it is only used when exiting an ImageFile instance.

def __enter__(self) -> ImageFile:
def _close_fp(self) -> None:
def __exit__(self, *args: object) -> None:

I also moved get_child_images() from Image to ImageFile. It requires the use of self.fp, so this made sense to me, but feel to tell me it should stay in Image.

Believe it or not, all the other changes in this PR flowed out from there.