imghdr is deprecated and slated for removal in Python 3.13

[elibon99]

The imghdr module is deprecated in Python 3.11 (see here) and slated for removal in Python 3.13.

imghdr is used to determine image type:

PGPy/pgpy/constants.py

Lines 430 to 435 in 30a7571

	@classmethod
	def encodingof(cls, imagebytes):
	type = imghdr.what(None, h=imagebytes)
	if type == 'jpeg':
	return ImageEncoding.JPEG
	return ImageEncoding.Unknown # pragma: no cover

If just looking for the JPEG type, maybe a simple fix to get rid of imghdr would be to look at the signature in the first few bytes of the image data like:

...
# Check for JPEG signature (first three bytes are b'\xFF\xD8\xFF')
if imagebytes.startswith(b'\xFF\xD8\xFF'):
    return ImageEncoding.JPEG
return ImageEncoding.Unknown

?