Return 'next' version. Eg, next(1.2) is 1.2_

def next(self):
        """Return 'next' version. Eg, next(1.2) is 1.2_"""
        if self.tokens:
            other = self.copy()
            tok = other.tokens.pop()
            other.tokens.append(tok.next())
            return other
        else:
            return Version.inf