Missing something on exception handling in Python 3

[Skip Montanaro]

I have code in the pylockfile package that looks roughly like this
(apologies for the indentation - I'm growing to dislike Gmail for
typing code fragments):

try:
write_pid_to_lockfile(somefile)
except OSError as exc:
if conditions_i_can_handle:
do_other_stuff...
else:
raise LockFailed("Failed to create %s" % self.path)

Thinking I would just get the final exception (not the one I caught
originally), I was surprised to see this output on my screen:
Traceback (most recent call last):
File "lockfile/pidlockfile.py", line 80, in acquire
write_pid_to_pidfile(self.path)
File "lockfile/pidlockfile.py", line 163, in write_pid_to_pidfile
pidfile_fd = os.open(pidfile_path, open_flags, open_mode)
OSError: [Errno 13] Permission denied: '/tmp/skip/lock'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "lockfile/pidlockfile.py", line 94, in acquire
raise LockFailed("failed to create %s" % self.path)
lockfile.LockFailed: failed to create /tmp/skip/lock

When I rung the same code with Python 2.7, I get the exception output
I anticipate:
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "lockfile/pidlockfile.py", line 94, in acquire
raise LockFailed("failed to create %s" % self.path)
LockFailed: failed to create /tmp/skip/lock

It appears exception handling changed in Python 3. How do I suppress
the lower level OSError?

Thx,

Skip