tox: add "upcoming-deprecations" environment

This runs trial against the latest (git) Twisted and Foolscap, to give
us an early warning about anything we're currently doing that will
become deprecated in their next releases.

tox.ini

@@ -20,6 +20,17 @@ setenv =
 commands =
          python misc/build_helpers/run-deprecations.py --stderr=_trial_temp/stderr.log trial --rterrors {posargs:allmydata}
 
+[testenv:upcoming-deprecations]
+basepython=python2.7
+passenv = USERPROFILE HOMEDRIVE HOMEPATH
+setenv =
+         PYTHONWARNINGS=default::DeprecationWarning
+deps =
+     git+https://github.com/twisted/twisted
+     git+https://github.com/warner/foolscap
+commands =
+         python misc/build_helpers/run-deprecations.py --stderr=_trial_temp/stderr.log trial --rterrors {posargs:allmydata}
+
 [testenv:checkmemory]
 commands =
          rm -rf _test_memory