Breaking changes:
- upgrade to python 3.9, torch 1.11
- torchtext.legacy dependencies are completely replaced by torch.utils.data
- joeynmt/tokenizers.py: handles tokenization internally (also supports bpe-dropout!)
- joeynmt/datasets.py: loads data from plaintext, tsv, and huggingface's [datasets](https://github.com/huggingface/datasets)
- scripts/build_vocab.py: trains subwords, creates joint vocab
- enhancement in decoding
- scoring with hypotheses or references
- repetition penalty, ngram blocker
- attention plots for transformers
- yapf, isort, flake8 introduced
- bugfixes, minor refactoring