tests/test-gendoc-it.t

run-tests: replace '/dev/null' with os.devnull for Windows

Many of the *.py tests were dying on:

    File "c:\Users\Matt\projects\hg\hgdemandimport\tracing.py", line 27, in log
      _pipe = open(os.environ['HGCATAPULTSERVERPIPE'], 'w', 1)
  IOError: [Errno 2] $ENOENT$: '/dev/null'

#require docutils gettext

  $ $TESTDIR/check-gendoc it
  checking for parse errors