typing: add typing for dogstatsd

[KowalskiThomas]

What does this PR do?

This PR adds typing annotations for the dogstatsd package.

The only changes are typing comments, except for

• Adding self._telemetry_socket_kind = None so that it's always set (even if the try fails)
• Using getattr(e, "errno", None) == errno.EPROTOTYPE over e.errno == errno.EPROTOTYPE since e is not properly typed to have errno and it's not easy to do it in a Py2-and-Py3 compatible way.
• Adding a cast to an Optional[float] value passed to a function expected float; reason is that "we know" it can only be None in certain cases and not in that specific one but the type checker can't know it statically (see comment)