Psd-tools3

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- add channel size checking in topil;
- add mlst metadata decoding;
- fix key collision issue in descriptor;
- performance improvement for packbit encoding/decoding;
- drop cython dependency in travis config;
- implement thumbnail, is_group, and parent methods in PSDImage.

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- major API changes;
- package name changed to `psd_tools2`;
- completely rewritten decoding subpackage `psd_tools2.psd`;
- improved composer functionality;
- file write support;
- drop cython compression module and makes the package pure-python;
- drop pymaging support.

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- composer alpha blending fix;
- documentation fix.

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- support cinf tagged block.

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- add missing extra image resource block signatures.

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- move psd_tools tests under tests/psd_tools.