ycb_django_pr16333

what a new run launched now would send

UserCreationForm should save data from ManyToMany form fields
Description
	
When using contrib.auth.forms.UserCreationForm with a custom User model which has ManyToManyField fields, the data in all related form fields (e.g. a ModelMultipleChoiceField) is not saved. 
This is because unlike its parent class django.forms.ModelForm, UserCreationForm.save(commit=True) omits to call self.save_m2m(). 
This has been discussed on the #django-developers mailing list ​https://groups.google.com/u/1/g/django-developers/c/2jj-ecoBwE4 and I'm ready to work on a PR.

Work only inside this repository checkout. Make the code change the task
describes, keeping the diff focused — no drive-by refactors.

When you are done, leave your changes committed or in the working tree;
they are collected automatically.

Stay on this snapshot checkout (`task/ycb_django_pr16333`). Never checkout, pull, or rebase onto `main`. That branch is a README-only orphan.
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