Mailgun2

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- Convert all docs from reStructuredText to Markdown
- Various cleanup
- Switch from Codeship to Drone CI
- Test dependency updates

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- Drop python 2 support
- Add mypy type checking

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- Refactor a lot of dependencies and packaging
- Update dependencies
- Better python 3 support

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Thanks to rjmackay

- Support mailgun tags
- Officially support python 3
- Switch to CodeClimate test coverage

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Thanks to fatisar:

- BREAKING: changed initialization to use public/private key
- Update to use mailgun v3 endpoints
- Add several new endpoints
- Update readmes

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- Convert docs to ReStructuredText
- Loosen requests package requirements