Several small updates

• add .coverage to .gitignore. it is created when I run tox
• update minimum testing version to 3.8 (observer.py imports typing.Protocol which is not in Python 3.7 #369)
• refactor iterator.py for https://www.flake8rules.com/rules/E731.html
• update Makefile
  • comment out pip-tools part: which seems not to be used ever
  • exclude .tox from linting
• Update requirements-dev.txt
  • Add black and isort: this has been originally in Makefile.
  • update versions
• integrate .venv and venv into venv

In addition, I think it is helpful for contributors if there is info of below.
From root files, I found the way to start developing this repo is

git clone git@github.com:faif/python-patterns.git
cd python-patterns
python -m venv venv
pip install --upgrade pip
make pyupgrade

And after coding, check it by this.

make pylinter
tox