Andy Burnham has refused to rule out a pact with the Green Party if he replaces Sir Keir Starmer as Prime Minister.The Mayor of Greater Manchester is contesting the Makerfield by-election on June 18 in a bid to become a Labour MP for the first time since 2017.If successful, Mr Burnham will then take on the Prime Minister as he vies for the Labour leadership.However, he faces stern opposition from Reform UK, who remain the most popular party in B…