Using getxmp from Pillow(min Pillow 8.3.0)

[bigcat88]

Changes proposed in this pull request:

• Minimum required Pillow increased from 6.2.0 to 8.3.0
• getxmp used from Pillow

For version 0.9.0

[codecov]

Codecov Report

Merging #55 (3a8bd4f) into master (3c60029) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #55   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           13        13           
  Lines         1128      1098   -30     
=========================================
- Hits          1128      1098   -30     

Impacted Files	Coverage Δ
pillow_heif/misc.py	100.00% <ø> (ø)
pillow_heif/__init__.py	100.00% <100.00%> (ø)
pillow_heif/as_plugin.py	100.00% <100.00%> (ø)