By Doyinsola Oladipo and Gigi Zamora NEW YORK (Reuters) -The number of people traveling by car and airplane for the Fourth of July holiday is expected to climb near pre-pandemic levels, a test for U.S. airlines that have struggled with inadequate staffing and flight cancellations this summer. The easing of COVID-19 restrictions and bottled-up travel demand are translating into the strongest summer since the pandemic for U.S. carriers. But so far the season has been marred by chaos, with more than 20,000 flight cancellations as U.S. airlines struggle with pilot staffing shortages that have pers…