Now that we can assume Python 3.6+, we can use the
email.headerregistry module to replace hacky manual email address
parsing.
Signed-off-by: Anders Kaseorg <anders@zulip.com>
This fixes having incompatible types of different items in
the dictionary, and makes `**identity_args` type-safe.
Signed-off-by: Zixuan James Li <p359101898@gmail.com>