New project structure

New project structure using pyproject.toml, setuptools_scm, src layout, and Github actions.

• Moved from setup.py to pyproject.toml, still using setuptools with setup.cfg.
• Moved to src layout, i.e. source files are now in src/ncvue directory.
• Use automatic versioning with setuptools_scm.
• Use Github actions for continuous integration instead of TravisCI.