Suddenly, pubcasting is in for a severe talking-to, if not a whupping. The House subcommittee that held such a congenial hearing on CPB’s long-overdue reauthorization a fortnight earlier is now preparing a second hearing July 20 to take pubcasters to task for swapping donor mailing lists with the Democratic Party. House Republicans were angry last week when they learned that Boston’s WGBH did it this spring, and angrier when they heard there were other times. And tempers will rise as similar reports come in from other stations. WNET in New York and WETA in Washington told reporters late last week that they’ve traded lists with both Democratic and Republican groups.