Update PyPI package homepage

[McRaeAlex]

Currently the homepage in PyPi https://pypi.org/project/py-allspice/ points at py-gitea.

We should change this so users who wish to file bugs or interact with us are pointed to the correct place.

Note: some links on the page are broken as well.

[shrik450]

The homepage was fixed in #42 but I missed the broken relative link on the examples directory. There's an open issue on pypa about this: pypa/readme_renderer#163, and changing it to be absolute and point to GitHub breaks the use case where you can click on it in your editor to go to the examples directory. I changed it to absolute anyway since it's more likely that people click on it from pypi than in an editor, and I snuck the change into #45.