Read the defined file, parse and set Version line, and return generator of filenames.

def _parse_manifest(self):
        """
        Read the defined file, parse and set Version line, and return generator of filenames.
        """
        with open(self.manifest_file, 'r') as f:
            for line in f:
                if line.startswith("Version"):
                    self.mver, self.relnum = self._parse_version(line)
                lineitems = line.split('  ')
                if len(lineitems) == 2:
                    asset_url = self._get_url(lineitems[1][:-1])
                    yield grequests.get(asset_url, stream=True)