Yaspin

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* API improvements: ``spinner``, ``color``, ``on_color``, ``attrs`` and ``side`` argument values are handled via ``__getattr__``
* New ``yaspin`` arguments: ``on_color``, ``attrs``
* ``right=False`` argument replaced with ``side="left"``
* ``Yaspin.right`` replaced with ``Yaspin.side``
* ``reverse`` argument replaced with ``reversal``
* ``Yaspin.reverse`` replaced with ``Yaspin.reversal``
* Remove default text stripping in ``Yaspin._freeze``

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* Add support for Python 3.7
* Drop support for Python 2.6 and 3.3

* dev: Migrate to Pipfile
* dev: Speedup local unittests with pytest-xdist

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* fix(16): remove default text stripping in ``Yaspin.write`` to allow printing of the hierarchical text

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* Update cli-spinners to v1.3.1

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* New ``hide`` and ``show`` methods to toggle the display of the spinner

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* New ``write`` method for writing text into terminal without breaking the spinner