Move back to pip and pyproject.toml

[pchomik]

To simplify env setup I decided to move back to pip and pyproject.toml as a latest standard.

What was done:

• [x] Changed pyproject.toml to support pip
• [x] Changed CI system to do not use poetry
• [x] Remove support for Python 3.7
• [x] Changed CI to build and deploy package on Python 3.8, on lowest support version
• [x] Updated documentation about pyproject.toml
• [x] Updated pre-commit hooks to latest version