Convclasses

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* Add support for modifiers
* Add dataclass field name modifier
* Add support for generic types (ported from cattrs)

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* Removed Python 3.6 support
* Added Python 3.9 support

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* Rename ``cattrs`` into ``conclasses``
* Move convclasses from ``attrs`` usage onto ``dataclasses``
* Fix incorrect structuring/unstructuring of private fields
* Change ``pendulum`` in docs onto ``arrow``

cattrs history
==============

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* Python 3.8 support.

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* Python 3.7 support.

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* The disambiguation function generator now supports unions of ``attrs`` classes and NoneType.