Evaluate markers for req against each extra that
        demanded it.

        Return False if the req has a marker and fails
        evaluation. Otherwise, return True.

def markers_pass(self, req, extras=None):
        """
        Evaluate markers for req against each extra that
        demanded it.

        Return False if the req has a marker and fails
        evaluation. Otherwise, return True.
        """
        extra_evals = (
            req.marker.evaluate({'extra': extra})
            for extra in self.get(req, ()) + (extras or (None,))
        )
        return not req.marker or any(extra_evals)