Gocept.collmex

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- Nothing changed yet.

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- Update invoice items to current collmex field names.
(`27 <https://github.com/gocept/gocept.collmex/pull/27>`_)

- Add CustomerAgreement (CMXCAG)

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- Mark wheel as not universal.

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- Drop support for Python 2.7, 3.5, 3.6.

- Add support for Python 3.9, 3.10, 3.11.

- Avoid password exhaustion by using invalid username for tests.

- Fix the tests to use fewer connections.

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- Drop support for Python 3.3 and 3.4.

- Add support for Python 3.6, 3.7 and 3.8b4.

- Migrate code to GitHub.

- Update tests to new Collmex URLs.

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- Implemented CMXABO, only applicable for "collmex Verein".