irnetbox.py: Add missing import

Commit fa65fafe was supposed to "fix connection retry on Linux", but in
fact broke it for everyone. In the next commit I'm going to make sure
this never happens again.
"make check" runs pylint static analyser on all python files

pep8 will catch minor annoyances like trailing whitespace;
pylint is the more important one, and will catch typos, even checking
the member names of other modules that you reference.

This should catch errors like the one fixed in commit 019fba2.

This requires pylint ("yum install pylint" on Fedora, "port install
py27-pylint on OS X", etc).

I used "pylint [some options] --generate-rcfile" to dump the specified
options to a config file. I did this with and without the options I
cared about (errors-only, output-format=parseable, include-ids=yes) and
diffed the two. Note that I still have to pass "--errors-only" on the
command line, because the equivalent config of "enable=E1102,E1103,..."
didn't seem to work: pylint still printed Cxxxx and Wxxxx warnings. You
might wonder why bother with a config file at all, and just pass
everything on the command line: It's to avoid the message "No config
file found, using default configuration" being printed once for every
file processed.

I selectively disable some overly-onerous errors:

* pylint's E1101 doesn't understand namedtuples. It reports:

    stbt.py:100: [E1101, detect_match] Instance of 'MatchResult' has no
    'match' member

* pep8's E203 (no whitespace around ":") and E225 (whitespace around
  operators) are inconvenient for the arithmetic (like "0.1 * 2**i") and
  array slicing (like "buf[2 : 3+data_len]") used in irnetbox.py.

* pep8's E251 (no whitespace around a keyword parameter's "=") and
  E501 (line < 80 chars) interfered with the config file specified
  inline as a string in irnetbox.py's unit test
  test_that_parse_config_understands_redrat_format.