Replace now-deprecated imghdr module

[MaggieFero]

Replace now-deprecated imghdr module

[Minnozz]

Pillow (an existing dependency) supports detecting image types by opening the image and looking at the format property: https://pillow.readthedocs.io/en/latest/reference/Image.html#PIL.Image.Image.format

Both places in the code where this functionality is used (that I could find), the purpose is to generate a random filename with an extension that matches the encoding of the image:

bookwyrm/bookwyrm/views/books/books.py

Lines 156 to 162 in 9e7b040

	try:
	image_content, extension = get_image(url)
	except: # pylint: disable=bare-except
	return None
	if not image_content:
	return None
	image_name = str(uuid4()) + "." + extension

bookwyrm/bookwyrm/models/fields.py

Lines 505 to 510 in 9e7b040

	image_content, extension = get_image(url)
	if not image_content:
	return None
	image_name = f"{uuid4()}.{extension}"
	return [image_name, image_content]

Using Pillow this way would parse the entire image instead of just the header, so that makes it less efficient.

[alcarithemad]

PEP 594 was updated to include recommendations to replace imghdr.

puremagic looks like it should be a suitable replacement.

get_image already reads the entire file, so overhead compared to imghdr should be minimal (puremagic may try to examine a few more kilobytes than imghdr's 32).