Movie and TV heavyweights including Oprah Winfrey, Meryl Streep, Natalie Portman and Lena Dunham are among those to sign an open letter calling for action against gender inequality. The 140 signatories from the entertainment arena are putting political leaders “on notice” in the letter which is spearheaded by international charity ONE. Also among those to sign are Black Panther stars Letitia Wright and Chadwick Boseman, Reese Witherspoon, Ryan Reynolds, Amy Schumer, Issa…