Silicon Valley’s quest to improve the diversity of tech company employees appears to have gotten a boost from the pandemic. Last year, Twitter’s Black staff increased by more than a third, to 9.4% of its U.S. workforce, while Latinx employees rose by almost half, to 8% of its U.S. staff, the company said Thursday. Twitter, which has 7,000 workers globally, the majority of which are in the U.S.,...