Folia

Re-release after minor fix in test suite; previous release was a bit premature.

* Adapted for FoLiA v2.4.1: strip whitespace left and right if there is only a sole string (proycon/folia88)

Implemented a ``move()`` method alongside ``copy()``, which does no deep copy.

* Implemented modality annotation (proycon/folia86)
* Revised external annotation (proycon/folia87)
* Properly handle removal of markup annotation (proycon/foliatools21)

* Implements FoLiA v2.3
* Adds support for the explicit form serialisation (proycon/folia84)
* Bugfixes
* also serialize when ``confidence=0`` 23
* fix missing t-ref/id attribute 19
* also perform text validation on string elements 15
* Implemented extra checks for spurious text
* added requests library as an explicit dependency
* fixed tests for new xmldiff version

Minor fix:
* no hard fail on missing version, but assume an old version instead 17