Products.externalmethod

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- Packaging cleanup

- Add support for Python 3.8

- Linting and code coverage configuration for ``tox``

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- Update to Bootstrap ZMI requiring `Zope >= 4.0b6`.

- Add support for Python 3.7.

- Drop support for Python 3.4.

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- Add support for Python 3.4, 3.5 and 3.6.

- Add compatibility with Zope 4.

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- Remove HelpSys support.

- Remove ZODB3 as direct dependency. Now we are able to use ZODB 4.0
and it is a dependency of Zope2 anyways

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- Handle both `func_code` / `__code__` and `func_defaults` / `__defaults__`.

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- Released as separate package.