Handle Python 3's O_CLOEXEC default

In Python 2, a file opened by a process was inherited by all of its
children.  Python 3 sets the O_CLOEXEC flag on the file descriptor
by default so this doesn't happen.  Because of the Python 2
behavior, our test code explicitly closes the file descriptor in
one test, but we shouldn't be doing that since it wouldn't happen
in a real usage scenario.

Fortunately, it appears both cases work in a sane fashion anyway.
With the explicit release removed from the child process, the py2
tests still run fine, and with that done the py3 tests no longer
have a problem either.  This is fortunate because if the py2
behavior allowed multiple processes to hold the lock at once it
would obviously be a Bad Thing. :-)

Change-Id: I133334dee068984b1309180c5338ac19d9934274