properly handle ipv6 hosts

[vchudnov-g]

We need to ensure our code works with IPv6. One aspect I noticed is the following:

We have core that defaults the host to use HTTPS. It looks like this:

        if ":" not in host:
            host += ":443"  # pragma: NO COVER

However, IPv6 addresses are allowed in URLs, enclosed in brackets (ref), and IPv6 addresses use colons as separators. The code as written would never default to HTTPS for IPv6. So we should have more robust logic to allow for that.

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Separately, the code above is preceded by a regex to look for the URL schema, but it only recognizes HTTP and HTTPS. I suggest we be more general and look for arbitrary schema by examining the host name until the leading colon. This is probably a pedantic point, but there are unregistered schemes being used, and more schemes could conceivably be registered. On principle, we shouldn't be more restrictive than we need to be.