Turns out that the library does a better job of parsing URIs than our
reinvented wheel. Who knew.

There are two things going on here. The first is that, unlike
parse_server_name, URI.fromBytes will strip off square brackets from IPv6
literals, which means that it is valid input to ClientTLSOptionsFactory and
HostnameEndpoint.

The second is that we stay in `bytes` throughout (except for the argument to
ClientTLSOptionsFactory), which avoids the weirdness of (sometimes) ending up
with idna-encoded values being held in `unicode` variables. TBH it probably
would have been ok but it made the tests fragile.

This is going to get too big and unmanageable.

We don't want to be doing .well-known lookups on these guys.

…ll-known

two reasons for this. One, it saves a bunch of boilerplate. Two, it squashes
unicode to IDNA-in-a-`str` (even on python 3) in a way that it turns out we
rely on to give consistent behaviour between python 2 and 3.