Add a Continuous Delivery pipeline for the PyPI package

[diegoferigo]

Publishing the PyPI package to gym-ignition-models is still done manually. It's not a big problem since this repo is not having a high number of merged PR, but it is a process that can be easily automatized.

Right now the operations to do are the following:

1. Merge the new PR to master
2. Make sure the local git repo is clean by stashing all the content (it affects the PyPI package naming)
3. Execute python3 setup.py sdist to create the dist/gym-ignition-models-<...>.tar.gz archive
4. Push it to PyPI with twine: twine upload dist/gym-ignition-models-<...>.tar.gz