Version 0.22.0

Release version 0.22.0

Added
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* Add mode "x" for SevenZipFile (#588)
* Add SevenZipFile#namelist method (#600)

Fixed
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* Append mode on non-existent files (#604)
* Fix NUMBER encoding of integer when 8 bytes(#591)

Changed
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* Minimum required Python version to be Python 3.8 (#601)
* Remove pyannotate from pyproject.toml (#598)

Document
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* Update user guide (#596)