Release for spaCy v3

This release makes spacy_conll compatible with spaCy's new v3 release. On top of that some improvements were made to make the project easier to maintain.

• [general] Breaking change: spaCy v3 required (closes #8)
• [init_parser] Breaking change: in all cases, is_tokenized now disables sentence segmentation
• [init_parser] Breaking change: no more default values for parser or model anywhere. Important to note here that
  spaCy does not work with short-hand codes such as en any more. You have to provide the full model name, e.g.
  en_core_web_sm
• [init_parser] Improvement: models are automatically downloaded for Stanza and UDPipe
• [cli] Reworked the position of the CLI script in the directory structure as well as the arguments. Run
  parse-as-conll -h for more information.
• [conllparser] Made the ConllParser class available as a utility to easily create a wrapper for a spaCy-like
  parser which can return the parsed CoNLL output of a given file or text
• [conllparser,cli] Improvements to usability of n_process. Will try to figure out whether multiprocessing
  is available for your platform and if not, tell you so. Such a priori error messages can be disabled, with
  ignore_pipe_errors, both on the command line as in ConllParser's parse methods