Update Portalbase to use with so close() isn't needed

Portalbase hasn't received the same treatment that adafruit_requests has with Connection Manager using `with` and still requires explicit `close()` for most cases. Needs to have the adafruit_requests treatment for `fetch` and other functions so explicit close isn't necessary to remember. It would be nice to have Portalbase take care of closing the connection automatically after use. It could help with cutting down retries, failed connections, and crashes from too many sockets open.

Compare Portalbase's fetch and other functions with the adafruit_requests implementation that uses Connection Manager; the issue names these as the starting points. Done means Portalbase closes connections automatically after use so callers no longer need to remember explicit close().

Written by the indexing model from the issue text.