Refactor codebase to improve PEP8 compliance

Refactor the codebase to PEP8 standard to improve code readability, consistency and maintainability.

### [File and directory names](https://peps.python.org/pep-0008/#package-and-module-names)

* short
* descriptive
* all-lowercase
* underscores can be used in the module name if it improves readability but no other special characters

E.g: `DAG-this-is-example.py` -> `dag_example.py`