python: added check-pylint makefile target and fixed the issues

Run pylint on Python code and fix the issues it found and silence the false
positives that came up.  Specifically we need to disable warnings on unused
variables when calling OMcache._request() which allocates C objects which
are not otherwise tracked in the Python side and would end up being garbage
collected before we use them.
omcache.py: handle missing responses gracefully

omcache_command() and omcache_io() may return without responses if the
request times out (or if we have a bug in omcache.c).  Handle these
gracefully by raising NotFoundError in get() and CommandError in all other
cases instead of failing with IndexError in list()[0].
test_suspended_memcache: add more tries to value fetching after resuming

try to read the value after resuming memcached a few more times as the calls
can fail a couple of times before the connection has been recreated and
confirmed as working.