workflows updated:
fixed a bug with undefined os in tests.yml and codestyle.yml
removed black-formatter
black-formatted code and flake8-checked:
every .py file was black-formatted and then checked with flake8
python 3.6 and 3.7 features removed
removed f-Strings feature like print(f'{}') (Python 3.6+ feature)
removed dataclasses (Python 3.7+ feature), use NamedTuple instead of it
print(f'{}')(Python 3.6+ feature) removed dataclasses (Python 3.7+ feature), use NamedTuple instead of it