tests/test-gendoc-it.t

tests: check availability of pyflakes by trying to import pyflakes module

Since e397c6d74652, we use the pyflakes module instead of the pyflakes
executable. As was pointed out during the review, the hghave check can be
rewritten to try to import the pyflakes module instead of spawning a new
subprocess.

#require docutils gettext

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