Dumping negative ImageIFD.TimeZoneOffset causes struct.error: argument out of range

[davetapley]

Per ISO/DIS 12234-2:

Time Zone Offset (in hours) of DateTimeOriginal tag-value relative to Greenwich Mean Time

and:

The allowed values are -12 to +11.

but:

Piexif/piexif/_exif.py

Line 104 in 3422fbe

34858: {'name': 'TimeZoneOffset', 'type': TYPES.Long},

then:

Piexif/piexif/_dump.py

Lines 206 to 209 in 3422fbe

	elif value_type == TYPES.Long:
	length = len(raw_value)
	if length <= 1:
	value_str = _pack_long(*raw_value)

and:

Piexif/piexif/_dump.py

Lines 173 to 174 in 3422fbe

	def _pack_long(*args):
	return struct.pack(">" + "L" * len(args), *args)

where L is unsigned long, causing ⬇️ for dump with any negative offset.

struct.error: argument out of range

[davetapley]

Fix for this will go in after #122