Packtools

* Initial support for SciELO PS 1.6.

* This release is purely bureaucratic, because Pypi does not allow a package
to be fixed and resubmitted with the same filename.

* Fixes syntax error on HISTORY.rst that caused style problems on Pypi.

* XML catalog to resolve system ids of type URL
[https://github.com/scieloorg/packtools/issues/110].
* Remove the use license restrictions from the Brazil instance
[https://github.com/scieloorg/packtools/issues/112].
* Make built-in schematron schemas available through the prefix ``:
scielo-br, sps-1.1, sps-1.2, sps-1.3, sps-1.4, sps-1.5.
* Better exception and log messages.

* Fixes a bug that would cause invalid product types to be accepted on
``article/front/article-meta/product/product-type``.
* Fixes a bug that would cause invalid invalid values to be accepted on
``article/front/article-meta/aff/institution/content-type``.
* Clean up unused attributes from classes from the ``packtools.style_error``
module.
* Log messages are now omitted from the stderr by default.
* Fixes a bug that would cause tests to fail on Python 3.6
[https://github.com/scieloorg/packtools/issues/107].

* Bugfix release
[https://github.com/scieloorg/packtools/issues/101].