Improve speed of get_task_family() (#3129)
Improve speed of get_task_family() (#3129)

* Use f-string instead of str.format()
* Save duplicate call to get_task_namespace()

Co-authored-by: Dillon Stadther <e4676899097b002b8d809044037857703176db8a@gmail.com>
dlstadther/luigi,spotify/luigi,dlstadther/luigi,riga/luigi,riga/luigi,spotify/luigi,dlstadther/luigi,spotify/luigi,spotify/luigi,riga/luigi,riga/luigi,dlstadther/luigi