Implement a PretixAPI class to interface with pretix

[marcoacierno]

For the association membership service we implemented a basic PretixAPI class: https://github.com/pythonitalia/pycon/blob/99432b3caef1fded4840f6e8275787322f2f55e7/backend/association_membership/handlers/pretix/api.py#L10

We should extend it and make it work for all operations the pycon backend needs