Return context manager for stream to send a (incremental) snapshot.

def send(self, targetPath, parent, diff, showProgress=True, allowDryRun=True):
        """ Return context manager for stream to send a (incremental) snapshot. """
        if parent is not None:
            cmd = ["btrfs", "send", "-p", parent, targetPath]
        else:
            cmd = ["btrfs", "send", targetPath]

        if Store.skipDryRun(logger, self.dryrun and allowDryRun)("Command: %s", cmd):
            return None

        process = subprocess.Popen(
            cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE, stdin=DEVNULL)
        _makeNice(process)

        return _Reader(process, process.stdout, targetPath, diff, showProgress)