* Specify `tls` extra for Twisted dependency.

It was already pulled in for us by `treq`, but we should be explicit
that we do use the `tls` functionality of Twisted directly.

* Mark `idna` as dev-dependency

This doesn't actually change anything, as `Twisted[tls]` will put it in
as a main dependency anyway.

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>
Co-authored-by: Brendan Abolivier <babolivier@matrix.org>