Changed list of single items dictionaries to dictionary

[radarhere]

Hi. Some suggestions for python-pillow#5144. Take them or leave them.

• Use context manager - It closes the image once the test is done with it
• Use isinstance to check type - It just seems simpler
• Check string value - To be more thorough
• Renamed variable - 'xmp_atribs' to 'xmp_attribs', being more consistent with 'element.attrib'
• Changed list of single item dictionaries to dictionary - meaning the test (and users) can refer to im.getxmp()["Description"]["Version"] instead of im.getxmp()["Description"][0]["Version"], which seems simpler, unless there's something I'm not aware of?

[UrielMaD]

Changes does look good for me, it's a good enhancement! Thanks!