Draftable-compare-api

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- Fix comparision expiry date not resepected
- Minor documentation update

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- **BREAKING CHANGE**: Drop support for Python 3.6 (minimum is now 3.7)
- Add support for comparison of text (`txt`) files
- Add support for export with cover page
- Miscellaneous developer tooling updates

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- **BREAKING CHANGE**: Drop support for Python 2.7 & 3.5 (minimum is now 3.6)
- Add support for Python 3.10
- Add type checking support & associated packages
- Use Python 3 language features with removal of Python 2 support
- Remove Python 2 compatibility code and `six` library
- Minor documentation updates
- Miscellaneous developer tooling updates

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- Add support for comparison export endpoint
- Add client tests against the Draftable API

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- Improve exception output on failing to infer file type
- `dr-compare`: Fix bug in calling `list|all` command

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- Add Python 3.9 support
- Use Black for code formatting
- Miscellaneous developer tooling updates