Move the source files to /src

[poncovka]

Clean up the project configuration and file structure:

• Move the source files of the dasbus package to the /src directory.
• Keep info about the Python package in the pyproject.toml file.
• Follow the latest Python packaging guidelines in the .spec file.
• Run the unit tests with pytest instead of unittest.
• Run tests on the installed package in Travis.

[jkonecny12]

A bit late but I like the changes here.