python-urwid-1.3.0 fails to build on x86_64 during the
"AsyncioEventLoopTest" test with the error "KeyError: '5 is not
registered'". It has failed repeatedly for some time now. It fails in
the same way when updated to python-urwid-1.3.1.

I looked for interesting changes made between the last successful build
and the first failing build. Notably, this range includes the upgrade
from python-3.3.5 to python-3.4.3 (08c04509). Asyncio was integrated
into the Python standard library in 3.4 — previously it had been an
external library. [0] Our python-3.4.3 package passes its 'test_asyncio'
test, FWIW.

I entered the failed build tree and successfully ran the tests using the
python-3.4.3-7 [1] installed by Debian Stretch. That only tells us so
much, but I think it does indicate either a bug in our python-3.4.3, or
some problem with python-urwid caused by the unfamiliar Guix build
environment.

Here's the hydra.gnu.org page:
http://hydra.gnu.org/build/861615

Here's the failing part of the build log:

--8<---------------cut here---------------start------------->8---

======================================================================
ERROR: test_remove_watch_file (urwid.tests.test_event_loops.AsyncioEventLoopTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/gnu/store/13n8xbi9wv9pigfyhir007qadr81jq46-python-3.4.3/lib/python3.4/asyncio/selector_events.py", line 234, in add_reader
    key = self._selector.get_key(fd)
  File "/gnu/store/13n8xbi9wv9pigfyhir007qadr81jq46-python-3.4.3/lib/python3.4/selectors.py", line 182, in get_key
    raise KeyError("{!r} is not registered".format(fileobj)) from None
KeyError: '5 is not registered'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/tmp/nix-build-python-urwid-1.3.0.drv-0/urwid-1.3.0/build/lib.linux-x86_64-3.4/urwid/tests/test_event_loops.py", line 33, in test_remove_watch_file
    handle = evl.watch_file(5, lambda: None)
  File "/tmp/nix-build-python-urwid-1.3.0.drv-0/urwid-1.3.0/build/lib.linux-x86_64-3.4/urwid/main_loop.py", line 1263, in watch_file
    self._loop.add_reader(fd, callback)
  File "/gnu/store/13n8xbi9wv9pigfyhir007qadr81jq46-python-3.4.3/lib/python3.4/asyncio/selector_events.py", line 237, in add_reader
    (handle, None))
  File "/gnu/store/13n8xbi9wv9pigfyhir007qadr81jq46-python-3.4.3/lib/python3.4/selectors.py", line 402, in register
    self._epoll.register(key.fd, epoll_events)
PermissionError: [Errno 1] Operation not permitted

----------------------------------------------------------------------
Ran 284 tests in 0.384s

FAILED (errors=1)
phase `check' failed after 4.6 seconds
--8<---------------cut here---------------end--------------->8---

[0]
https://docs.python.org/3/library/asyncio.html

[1] Reported by `apt-cache show python3`, this python-3.4.3-7's .deb has
a SHA256 hash of:
53fa197ee35501152b1897bf84ab6123f7f65201efdddc2e4aa882de494f3870