Rnc

1. Setting language in the parallel corpus updated, new values set.
2. Kwarg `lang` added to ParallelCorpus.
3. Bumpversion added.
4. Dependencies updated.

1. Asyncio support added, some methods implemented:
1. `corp.request_examples_async()`
2. `MultimodalCorpus.dump_all_async()`
3. `MultimodalExample.dump_file_async()`
2. Use poetry instead of `setup.py`; dependencies updated.
3. Request validation fixed according to new ruscorpora.ru page structure.
4. Python 3.9 support added.

...

1. Log message format changed, 34 closed.
2. Custom exceptions created, 36 closed.
3. ABC used, 37 closed.
4. Log message added, 39 closed.
5. Requirements version updated according to security vulnerability.
6. `Corpus.findall()/finditer()` fixed
7. Docs and logging improved.
8. Setting on GitHub added: issue templates etc.
9. Parsing `MultimodalCorpus` fixed.

* Quickfix: deepcopy removed

Changed
* Logging improved.
* Use workers and queue instead of aiojobs. Issue 26 closed.
* Versions of requirements specified. Issue 27 closed.
* Email changed. Issue 28 closed.
* Other performance improvements.

Changed
* Docs were improved.
* Params validating was added. Issue 17 closed.
* Encoding was changed from `utf-16` to `utf-8`. Issue 16 closed.
* `findall` and `finditer` implemented. Issue 19 closed.
* Using `ujson` instead of `json`. Issue 15 closed.
* Some useless validating requests removed. Issue 21 closed.