Python-rest-framework

Fix new setup.py config

* Added [`base views`][BaseViews]
* Added [`base views mixins`][ViewsMixins]
* Added [`views pagination`][ViewsPaginations]
* Added [`ApiException`][ApiException]
* Added [`views for Flask`][FlaskViews]
* Added [`views for AioHttp`][AioHttpViews]
* Added [`views for Sanic`][SanicViews]
* Added [`utils.decorators`][utils_decorators] for class check attributes and signature
* Fix not actual docs

[BaseViews]: rest_framework/views/base.py
[ViewsMixins]: rest_framework/views/mixins.py
[ViewsPaginations]: rest_framework/views/paginations.py
[FlaskViews]: rest_framework/views/flask
[AioHttpViews]: rest_framework/views/aiohttp
[SanicViews]: rest_framework/views/sanic
[ApiException]: rest_framework/exceptions.py
[utils_decorators]: rest_framework/utils/decorators.py

Added [ChoiceValidator](./rest_framework/serializers/validators.py231) in [serializers.validators](./rest_framework/serializers/validators.py)

* Mini fix `Serializers` `_field_validation` method.
* Add `BooleanNullField`.
* Remove Null values from `BooleanField`.

Mini fix `SerializerMethodField` call `to_representation` method