Updates needed in "Pushing to pip" section of "packages" tutorial

in addition to changes re: TestPyPI from #68:
- command to build package should be changed to to `python setup.py sdist bdist_wheel` so wheel binary is also created (see [PEP 427](https://www.python.org/dev/peps/pep-0427/))
- encourage running `twine check dist/*` before uploading
- upload command should be changed to `twine upload dist/*` so `.whl` file is available on PyPI
- replace link to article at bottom with link to [this tutorial](https://realpython.com/pypi-publish-python-package/#publishing-to-pypi) instead