Require twisted (>=16.1.0) all the time.

The core Wormhole exchange will retain a blocking/non-Twisted
implementation, but the file-transferring Transit class is going to be
Twisted-only (maybe Twisted+asyncio). I want to improve the
protocol (add more parallelism, reduce round-trips), and the blocking
implementation is a messy bundle of threads and ick.

When this process is done (eventually), I'll be splitting out the
blocking Wormhole class into a separate distribution, which doesn't
depend upon Twisted.

setup.py

@@ -21,10 +21,8 @@ setup(name="magic-wormhole",
       entry_points={"console_scripts":
                     ["wormhole = wormhole.scripts.runner:entry"]},
       install_requires=["spake2==0.3", "pynacl", "requests", "argparse",
-                        "six"],
+                        "six", "twisted >= 16.1.0"],
       extras_require={"tor": ["txtorcon", "ipaddr"]},
-      # for Twisted support, we want Twisted>=15.5.0. Older Twisteds don't
-      # provide sufficient python3 compatibility.
       test_suite="wormhole.test",
       cmdclass=commands,
       )

tox.ini

@@ -7,12 +7,6 @@
 envlist = py27,py33,py34,py35
 skip_missing_interpreters = True
 
-# There's a race-condition bug in Twisted-15.5.0 (#8014, fixed in trunk) that
-# manifests as various intermittent magic-wormhole test failures that always
-# include twisted.internet.endpoints "iterateEndpoint" or "checkDone". So run
-# all builds with a copy of Twisted from git 'trunk' until Twisted-16.0.0 is
-# released and we can just depend on that.
-
 # On windows we need "pypiwin32" installed. It's supposedly possible to make
 # Twisted do this by depending upon "twisted[windows]" instead of just
 # "twisted", but when I try this via Appveyor, the extra is ignored.
@@ -24,7 +18,6 @@ skip_missing_interpreters = True
 
 [testenv]
 deps =
-    twisted >= 16.1.0
     pyflakes
     {env:EXTRA_DEPENDENCY:}
 commands =