Products.mailhost

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- Move GenericSetup export/import support from the GenericSetup package
to MailHost as a setuptools extra.

- Python 3 compatibility

- Require Zope 4, aka drop Zope 2.13 support.

- add test coverage reporting

- Use `implementer` class decorator.

- Drop long-deprecated support for uuencoded emails.

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- Remove HelpSys support.

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- Adjust tests to pass with latest zope.sendmail versions.

- Specify detailed distribution dependencies.

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- LP 642728: Fixed TypeError on nested multi part messages in MailHost.

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- Released as separate package.