Section 20A. No contract, whether written or oral, between any employee or prospective employee and his employer, prospective employer or any other person, whereby either party thereto undertakes or promises not to join or not to remain a member of some specified labor organization or any labor organization, or of some specified employer organization or any employer organization, and/or to withdraw from an employment relation in the event that he joins or remains a member of some specified labor organization or any labor organization, or of some specified employer organization or any employer organization or organizations, shall afford any basis for the granting of legal or equitable relief by any court against a party to such undertaking or promise.