This is PyPy's code of the pytest lib.  We don't expect to upgrade it
very often, but once we do:

    WARNING!

    WE HAVE MADE A FEW TWEAKS HERE!

Please be sure that you don't just copy the newer version from
upstream without checking the few changes that we did.  This
can be done like this:

    cd <this directory>
    hg log . -v | less

then search for all " _pytest/" in that list to know which are the
relevant checkins.  (Look for the checkins that only edit one
or two files in this directory.)