Folia

Expanded `fixinvalidreferences` option to also work at parse-time

* Remove control characters when normalizing text
* Fix text serialisation for empty table row with empty cell 28

[Sheean Spoel]
* Prevent tripping over XML comments 27

[Maarten van Gompel]
* Fix missing text delimiter output for empty cells and empty rows 28

[Maarten van Gompel]
* implement etymology annotation (proycon/folia105), corrected some old mentions of 'token annotation' to 'inline annotation'
* documentation: added missing subtoken classes (proycon/folia105)
* extra software metadata
* ignore XML processing instructions (rather than crashing on them) (proycon/foliatools55)
* documentation fixes

[Ko van der Sloot]
* updated for FoLiA v2.5.3
* added filename to error message

* Added support for a FOLIAPY_FORCE_LOCALSETDIR environment variable that disables remote downloads of set
definitions and looks locally instally instead. Enables off-line usage. https://github.com/proycon/flat/issues/182.
* Metadata update
* Dropped Python 3.6 support (requiring 3.7+)

Bugfix release