Branch deletion uses personal token instead of bot token

[KyleJu]

In our weekly update, we noticed branch deletions use the user token who originally setup pyup, rather than using its own bot account (which appears to be the thing used to create branches and pull requests)?

see @Hexcles Hexcles deleted the py-scheduled-update-2021-01-04 branch on Jan 11 on the bottom of the update

Any idea we can fix it? More details here, web-platform-tests/wpt.fyi#2339