Allow running twisted trunk against other branches (#15302)

* Allow running twisted trunk against other branches

I would like to do this so we can try Synapse's typechecking against a
specific branch that the project solicited tests for, see
https://mail.python.org/archives/list/twisted@python.org/message/GGO5JHA5S475AK6JZ3GCC3GIHGKQYM6Y/

* Changelog