tox upcoming-deprecations: emit foolscap/twisted versions

Note that Twisted doesn't do anything like Versioneer, so this will
generally show e.g. "Twisted-16.1.0" for the entire interval between
16.1.0 and 16.2.0.

tox.ini

@@ -29,6 +29,7 @@ deps =
      git+https://github.com/twisted/twisted
      git+https://github.com/warner/foolscap
 commands =
+         flogtool --version
          python misc/build_helpers/run-deprecations.py --warnings=_trial_temp/deprecation-warnings.log trial --rterrors {posargs:allmydata}
 
 [testenv:checkmemory]