Foliadocserve

* fix for problems after explicit saving (proycon/flat169)
* minor cleanup
* deprecated Python 3.6 and below

Minor bugfix release:
* Fixed git intialization

* Implemented visualisation of superscript and subscript styles (just an educated guess based on class names)
* use fixinvalidreferences parameter to automatically fix (=delete) invalid references from span annotation (proycon/flat174)
* Implemented support for FoLiA v2.5 white-space handling and support for explicit linebreaks 16

Bugfix release:

* Automatically fix documents with unassigned processors (fixunassignedprocessor)

* also propagate/serialize annotations inside str (proycon/flat92)

automatically convert document directory to an absolute path