Previously, TLS could only be used with STARTTLS.
Add a new option `force_tls`, where TLS is used from the start.
Implicit TLS is recommended over STARTLS,
see https://datatracker.ietf.org/doc/html/rfc8314Fixes#8046.
Signed-off-by: Jan Schär <jan@jschaer.ch>
Turns out that the functionality added in #10546 to skip TLS was incompatible
with older Twisted versions, so we need to be a bit more inventive.
Also, add a test to (hopefully) not break this in future. Sadly, testing TLS is
really hard.
