Tox doesn't run shell pipelines, and is unhappy about running tools that
it didn't install itself (including non-python things like "make"). So
this adds misc/build_helpers/run-deprecations.py, a python script that
runs a given command (i.e. trial) and writes stdout into a separate file
where the buildbot can see it, and counts the "DeprecationWarning" lines
from the file to decide the returncode.
This ought to improve the status display on the buildbot "test
deprecations" step.
