Check if repodata urls are valid.

def _check_repos(self, repos):
        """Check if repodata urls are valid."""
        self._checking_repos = []
        self._valid_repos = []

        for repo in repos:
            worker = self.download_is_valid_url(repo)
            worker.sig_finished.connect(self._repos_checked)
            worker.repo = repo
            self._checking_repos.append(repo)