Py7zr

- Update documentation, artwork and specification document.

Added
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* File format specification: add ISO/IEC standard style specification document.

Changed
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* Update password handling and drop get_password() helper (162)

Fixed
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* Now return correct header size by archiveinfo() method.(169)

Removed
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* Drop ArchiveProperty class: A field has already deprecated or not used.(170)
* Drop AntiFile property: a property has already deprecated or not used.

Changed
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* Enable encoded header and add more test with 7zip compatibility.(164)

Fixed
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* Disable adding CRC for encoded header packinfo.(164)

Fixed
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* Fix password leak/overwrite among SevenZipFile objects in a process.(159)
This can cause decryption error or encryption with unintended password.
* Release password on close()

Changed
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* Refactoring SevenZipFile class internals. (160)
* Refactoring classes in compressor module. (161)

Fixed
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* SevenZipFile.test() method now working properly. (155)

Changed
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* Add 'packinfo.crcs' field digests data when creating archive.(157)
It help checking archive integrity without extraction.
* CLI: help option to show py7zr version and python version.
* Use importlib for performance improvement instead of pkg_resources module.

Added
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* Support extra methods for archiveinfo() method.(150)