mypy: Improve MessageOutput typing in export.py.
mypy: Improve MessageOutput typing in export.py.

See the comments above for why this is the correct list of options.
rishig/zulip,brainwane/zulip,kou/zulip,kou/zulip,eeshangarg/zulip,hackerkid/zulip,shubhamdhama/zulip,kou/zulip,jackrzhang/zulip,rishig/zulip,zulip/zulip,timabbott/zulip,eeshangarg/zulip,timabbott/zulip,jackrzhang/zulip,rht/zulip,showell/zulip,zulip/zulip,synicalsyntax/zulip,rht/zulip,dhcrzf/zulip,tommyip/zulip,dhcrzf/zulip,rishig/zulip,synicalsyntax/zulip,dhcrzf/zulip,punchagan/zulip,punchagan/zulip,brainwane/zulip,kou/zulip,punchagan/zulip,brainwane/zulip,synicalsyntax/zulip,showell/zulip,brainwane/zulip,eeshangarg/zulip,rht/zulip,hackerkid/zulip,kou/zulip,tommyip/zulip,showell/zulip,shubhamdhama/zulip,brainwane/zulip,hackerkid/zulip,hackerkid/zulip,shubhamdhama/zulip,timabbott/zulip,kou/zulip,jackrzhang/zulip,eeshangarg/zulip,punchagan/zulip,rishig/zulip,synicalsyntax/zulip,hackerkid/zulip,jackrzhang/zulip,showell/zulip,shubhamdhama/zulip,timabbott/zulip,jackrzhang/zulip,zulip/zulip,synicalsyntax/zulip,synicalsyntax/zulip,dhcrzf/zulip,brainwane/zulip,andersk/zulip,punchagan/zulip,rht/zulip,tommyip/zulip,tommyip/zulip,zulip/zulip,timabbott/zulip,timabbott/zulip,punchagan/zulip,andersk/zulip,andersk/zulip,synicalsyntax/zulip,jackrzhang/zulip,brainwane/zulip,jackrzhang/zulip,eeshangarg/zulip,andersk/zulip,shubhamdhama/zulip,zulip/zulip,dhcrzf/zulip,tommyip/zulip,timabbott/zulip,kou/zulip,andersk/zulip,dhcrzf/zulip,rht/zulip,tommyip/zulip,rishig/zulip,shubhamdhama/zulip,andersk/zulip,showell/zulip,showell/zulip,tommyip/zulip,rht/zulip,rishig/zulip,dhcrzf/zulip,eeshangarg/zulip,zulip/zulip,zulip/zulip,rht/zulip,hackerkid/zulip,andersk/zulip,hackerkid/zulip,eeshangarg/zulip,shubhamdhama/zulip,rishig/zulip,punchagan/zulip,showell/zulip