Transformer-discord-notifier

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* ignore linkcheck with version tag (if tags have not been pushed it will fails)
* Blocking message deletion?

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* Github Actions - tox tests

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* Github Actions - pypi publishing

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* Add travis build jobs.
* Add coveralls coverage statistics.

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* Reintroduce tests with ``pytest`` and ``tox``.
* Add simple tests for :class:`~transformer_discord_notifier.discord.DiscordClient`.
* Add tests for :class:`~transformer_discord_notifier.transformers.DiscordProgressCallback`.

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* Let Discord bot gracefully handle initialization failures.
* Let transformer callback handler handle invalid configs gracefully, to simply exit.
* Better handling of edge cases of Discord client login.