settings: Fix typing for AUTHENTICATION_BACKENDS.
settings: Fix typing for AUTHENTICATION_BACKENDS.

shubhamdhama/zulip,kou/zulip,kou/zulip,hackerkid/zulip,showell/zulip,showell/zulip,brainwane/zulip,brainwane/zulip,rht/zulip,timabbott/zulip,tommyip/zulip,showell/zulip,showell/zulip,timabbott/zulip,punchagan/zulip,shubhamdhama/zulip,punchagan/zulip,eeshangarg/zulip,timabbott/zulip,synicalsyntax/zulip,kou/zulip,hackerkid/zulip,kou/zulip,rht/zulip,brainwane/zulip,eeshangarg/zulip,punchagan/zulip,zulip/zulip,brainwane/zulip,brainwane/zulip,synicalsyntax/zulip,eeshangarg/zulip,hackerkid/zulip,synicalsyntax/zulip,hackerkid/zulip,rht/zulip,showell/zulip,brainwane/zulip,punchagan/zulip,hackerkid/zulip,showell/zulip,synicalsyntax/zulip,zulip/zulip,andersk/zulip,tommyip/zulip,andersk/zulip,kou/zulip,shubhamdhama/zulip,showell/zulip,synicalsyntax/zulip,zulip/zulip,timabbott/zulip,zulip/zulip,synicalsyntax/zulip,kou/zulip,tommyip/zulip,shubhamdhama/zulip,punchagan/zulip,hackerkid/zulip,brainwane/zulip,rht/zulip,timabbott/zulip,andersk/zulip,eeshangarg/zulip,zulip/zulip,rht/zulip,synicalsyntax/zulip,shubhamdhama/zulip,rht/zulip,tommyip/zulip,eeshangarg/zulip,andersk/zulip,shubhamdhama/zulip,punchagan/zulip,rht/zulip,eeshangarg/zulip,eeshangarg/zulip,andersk/zulip,tommyip/zulip,zulip/zulip,andersk/zulip,kou/zulip,tommyip/zulip,timabbott/zulip,punchagan/zulip,shubhamdhama/zulip,timabbott/zulip,hackerkid/zulip,zulip/zulip,tommyip/zulip,andersk/zulip