Blazeform

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- Fix mutable default argument in tolist (556fcf0_)

.. _556fcf0: https://github.com/blazelibs/blazeform/commit/556fcf0

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- drop support for python 2
- modernize package setup and CI
- support python 3.8 (1d9afa9_)

.. _1d9afa9: https://github.com/blazelibs/blazeform/commit/1d9afa9

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* handle string type in file upload for blank submissions

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* update validation messages for consistency across python versions

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* added support for Python 3 (3.4 and 3.5)
* set up CI and coverage

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* make is_empty more general with respect to input type, 0 should not be empty