Improve mail transmission perf, invoice creation

[nspo]

Previously, pybarsys opened a new connection for each mail. With this change, it (for the most part) opens one connection and keeps that open for all mails.
This seems to improve perf by roughly 10x in some
simple tests, making it less frustrating to generate a lot of invoices.

If there is an error during mail tx, new invoices now are automatically deleted to not make it seem like they were delivered to the user.

There was a small validation bug which made it impossible to create a new user for which someone else has to pay in one go (how was this never a bug report?!).

Updated some dependency to make it compatible with recent Python versions. Actually, a lot more updates are needed...

[nspo]

fyi @Merlijnv

[jallmenroeder]

Looks good to me, but I did not test it myself. Maybe there's further improvement in submitting a list of all mails to the send_messages() function, instead of one message at a time. But I'm not sure if it's worth it.

[nspo]

Thanks!

Maybe there's further improvement in submitting a list of all mails to the send_messages() function, instead of one message at a time. But I'm not sure if it's worth it.

I don't think that should make a real perf difference because send_messages itself is just a for loop: https://github.com/django/django/blob/674eda1c03a3187905f48afee0f15226aa62fdf3/django/core/mail/backends/smtp.py#L137

and the heavy stuff in that function (establish connection) is already done/skipped with each call

I agree that a more extensive refactoring could have been nice though. But one disadvantage would've been that you just get one error for all of the (potentially) sent mails and don't know which were delivered and which were not. So it'd be an issue to decide whether you delete the local invoices if an error occurs because maybe 49/50 invoices were delivered correctly but the last one failed. Edit: never mind, the function returns the number of successfully sent mails 🙈 didn't see that when writing the code. But the first argument still stands