Small refactor of Builders/Builder.py (#273)

* refactor: changed Builder.py

* refactor: changed name from _BuilderState to BuilderState

* refactor: added unit tests for the BuilderStates module

* feat: added tox-based CI

* feat: removed the newest python version

* feat: removed unnecessary entries

* feat: simplified workflow

* fix: fixing executing tests in Actions

* fix: changed allow_externals from poetry to pytest

* fix: add pytest-cov as a required package for testing

* fix: removed macos and windows testing

* refactor: removed commented line