We remove the ABC metaclass as now 'normalize_tag' and 'normalize_tags'
are cyclically defined, and subclasses must redefine at least 1.

Later these will become part of the official System interface

Thanks to Alexandre Bernard for reporting this.
https://mailman-mail5.webfaction.com/pipermail/kwant-devel/2018-May/000088.html

Summary of changes:
* Add a test for trailing whitespace, remove all trailling whitespace
* Add a test for broken URLs in docs, fix all broken URLs and permanent redirects
* Switch sphinx doc build to strict (warnings to errors) and nitpicky (broken links to warnings). Fix all sphinx warnings.
* Deploy docs from any commit to [kwant/kwant](https://gitlab.kwant-project.org/kwant/kwant) to https://test.kwant-project.org/doc/dev
* Deploy docs from any commit to master at kwant/kwant to https://kwant-project.org/doc/dev
* Deploy docs from any tag release tag (so anything that is vX.Y.Z) at kwant/kwant to https://kwant-project.org/doc/X.Y