tests/test-gendoc-pt_BR.t

run-tests: check if stream is a tty before using color

Previous implementation (e80041832eec) checked only if sys.stderr was a tty
which was less general. Also makes sure that colors is never used if pygments is
not available, irrespective of --color flag value.

#require docutils gettext

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