Bump project to Python 3.11

[mariobehling]

Update the project to Python 3.11. Similar changes were done in Pretix here https://github.com/pretix/pretix/pull/2982/files

[weshaan]

still on? i can do this

[mariobehling]

Go ahead. No need to wait for anyone. Can just make a PR.

[hongquan]

There is another PR #36

But I marked it as WIP because we haven't tested eventyay-tickets with Python 3.10+ yet. With many 6 year old dependencies, I doubt they work with Python 3.10.

[hongquan]

In the meantime, someone please remove Python 3.6, 3.7 from GitHub Action script. GitHub seems no longer provide runners with these Python versions.

[Shivangi-ch]

In the meantime, someone please remove Python 3.6, 3.7 from GitHub Action script. GitHub seems no longer provide runners with these Python versions.

@hongquan I have fixed this issue in PR #51