Bpmappers

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- Add support Python 3.9
- Remove support Python 3.5
- Remove support Django 2.1

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- Add support Django 3.1
- Remove support Python 2.7, 3.4
- Remove support Django 1.11, 2.0
- Remove six dependency

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- Add support Python 3.8
- Add supports Django 2.2, 3.0

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- Modify documentation config

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- Support Python 3.7
- Support Django 2.0, 2.1
- Remove support old versions of Django <1.11.

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- Rewrote documentation.
- Change testing framework to pytest.
- Removed support old versions of Python <2.7, <3.4.
- Removed support old versions of Django <1.8.
- Removed SortedDict, mapping result uses OrderedDict.
- Removed MultiValueDict, replaced to defaultdict.