Licensing, data source fix, README and update for new Python

[MiranDaniel]

Licensing

Based on our previous discussion I have added licensing information to the project and the result images/PDFs.
I suggest the PolyForm Noncommercial License 1.0.0 license for the code and Creative Commons Attribution Non Commercial Share Alike 4.0 International for the results.

• PolyForm Noncommercial License 1.0.0 TL;DR
  • Reuse, modifying and sharing is allowed as long as the project is not used for commercial purposes
  • It's not a true open-source license but a source-available license.
  • You can read the full text here, shouldn't take more than 3 minutes: https://polyformproject.org/licenses/noncommercial/1.0.0/
• Creative Commons Attribution Non Commercial Share Alike 4.0 International TL;DR
  • Users must give appropriate credit when using the combo chart and they cannot use it for commercial purposes.
  • This license would allow someone to edit the chart as long as they share it again. I am not totally sure if this is what we want since someone could theoretically change the info maliciously and pass it off as something made by us. The same license but without remix rights exists though.

I have also added the license info into the chart itself right below the version/generation date.

README changes

New Python versions do not allow installing packages globally with pip and using a venv is recommended.
I have created a requirements.txt file that lists all the requirements and updated the README to match the new setup/usage guide.

Also decided to add a general footer to explain what the project is/what it does. Example image included!

Footer change

I have decided to add the date of the last combo data update to the footer right next to the generation date. Should make it more obvious to notice old data when generating.

Data source URL change

The old URL (https://github.com/TripSit/drugs/blob/main/combos.json) now returns the HTML page instead of raw JSON. My patch uses the raw file GitHub API instead. The data is still taken from the combos.json file on TripSit/drugs, the only thing that changed is the API used to access it.

[MiranDaniel]

Sorry for making such a complex PR, let me know if you'd like me to split it into multiple smaller ones per-issue.
I'd love to hear your thoughts on the licenses, I simply chose the ones I think you'll want but this is totally up to you.

[MiranDaniel]

Tiny question: How do you version this project? Will this PR be version 5.1.0?