exceptions: Fix AbstractEnum typing.
exceptions: Fix AbstractEnum typing.

This will be needed for mypy 0.730.

Signed-off-by: Anders Kaseorg <dfdb7392591db597bc41cf266a9c3bc12a2706e5@zulipchat.com>

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