MAINT: Prune all branches

[HaoZeke]

Most of the branches are dead weight. I would suggest removing all of them and keeping only main<->master, latest and a 'scipy' branch (which should track latest with additional changes needed in scipy).

[HaoZeke]

It is also rather difficult to determine which branch to target for PRs as well at the moment.

[HaoZeke]

Currently perhaps we can just have main (corresponding to what scipy needs) and latest which tracks upstream latest. We could also have upstream tracking master but I'm not sure it is necessary.