Seveno-pyutil

- removed xml_utilities
- removed model_utilities

- small fixes

- dropped support for Python versions < 3.5
- fixed small bugs
- added error_utilities
- added xml_utilities

* Refactored and improved SQLAlchemy logging capabilities of SQLFilter
* Added JSONEncoderWithDateTime
* Added IsoTimeField
* Fixed problems with Validateable, renamed to ValidateableMixin
* Added 'import_string' - ripped from Django, need it elsewhere so like to have it independent of Django

* Added `in_batches` function

* silent_remove now removes directories too.