Python-dogecoin

* Add package documentation via [MkDocs](https://www.mkdocs.org/)
* Add linting with [flake8](https://flake8.pycqa.org/en/latest/index.html)
* Add running unittests on [Github Actions](https://docs.github.com/en/actions)
* Fix issue with configuration loading

* Refactoring functions for reading configuration files
* Provide different `rpcport` fallback for `testnet` mode