Magic-wormhole

Mostly internal changes:

* twisted.transit was added, so Twisted-based applications can use it now.
This includes Producer/Consumer -based flow control. The Transit protocol
and API are documented in docs/transit.md .
* The transit relay server can blur filesizes, rounding them to some
roughly-logarithmic interval.
* Use --relay-helper="" to disable use of the transit relay entirely,
limiting the file transfer to direct connections.
* The new --hide-progress option disables the progress bar.
* Made some windows-compatibility fixes, but all tests do not yet pass.

* the server can now "blur" usage information: this turns off HTTP logging,
and rounds timestamps to coarse intervals
* `wormhole server usage` now shows Transit usage too, not just Rendezvous

* `wormhole` can now send/receive entire directories. They are zipped before
transport.
* Python 3 is now supported for async (Twisted) library use, requiring at
least Twisted-15.5.0.
* A bug was fixed which prevented py3-based clients from using the relay
transit server (not used if the two sides can reach each other directly).
* The `--output-file=` argument was finally implemented, which allows the
receiver to override the filename that it writes. This may help scripted
usage.
* Support for Python-2.6 was removed, since the recent Twisted-15.5.0 removed
it too. It might still work, but is no longer automatically tested.
* The transit relay now implements proper flow control (Producer/Consumer),
so it won't buffer the entire file when the sender can push data faster
than the receiver can accept it. The sender should now throttle down to the
receiver's maximum rate.

* Add key-confirmation message so "wormhole send" doesn't hang when the
receiver mistypes the code.
* Fix `wormhole send --text -` to read the text message from stdin. `wormhole
receive >outfile` works, but currently appends an extra newline, which may
be removed in a future release.
* Arrange for 0.4.0 senders to print an error message when connecting to a
current (0.5.0) server, instead of an ugly stack trace. Unfortunately 0.4.0
receivers still display the traceback, since they don't check the welcome
message before using a missing API. 0.5.0 and 0.6.0 will do better.
* Improve channel deallocation upon error.
* Inform the server of our "mood" when the connection closes, so it can track
the rate of successful/unsuccessful transfers. The server DB now stores a
summary of each transfer (waiting time and reported outcome).
* Rename (and deprecate) one server API (the non-EventSource form of "get"),
leaving it in place until after the next release. 0.5.0 clients should
interoperate with both the 0.6.0 server and 0.6.0 clients, but eventually
they'll stop working.

* properly require "setuptools" for install (47, jameshilliard)

* correctly close a mailbox which still has a nameplate (28)
* remove python2 support
* test on python 3.8, 3.9, 3.10, 3.11 and 3.12 series
* drop "six" (35)
* upgrade "versioneer"