Tri.form

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* Fixed coverage build (this also broke the travis build and as a consequence the pypi release process)

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* Rendering of submit button is now a `Link` and you can now supply your own buttons and links here.

* Fixed crash in rendering fields in some cases. This was due to an incorrect use of `format_html`.

* Flask and Jinja2 support

* Updated examples for modern django, and added an example

* Modernize attrs rendering on Field (this introduces a few deprecations)

* render_attrs support for style attributes. This means you can now do `Link(attrs__style__display='none')` for example.

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* File field tried to render the binary contents on form validation errors. This is a crash in Django 2.1

* `from_model` was broken when traversing a foreign key relation with `__`

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* Dropped Django 1.8 support

* Fixed deprecation warnings for Django 2.0

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* Added `Field.boolean_tristate` for editing boolean fields with null value allowed

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* Simplified `MISSING` definition