ROME — A few dozen black-clothed tour guides and tour organizers twirled white umbrellas to the tune of “Singing in the Rain” outside the Pantheon, one of Rome’s greatest tourist attractions recently. The Pantheon, an ancient Roman temple, was among a wave of attractions across Italy that reopened this month after the coronavirus lockdown. In the days after some of the first lockdown restrictions were lifted, Italians relished the empty streets, rediscovering city monuments and museums that they would normally avoid because of long lines. But even as travel restrictions are lifted throughout Europe, reluctance to travel outside national borders remains high. Forecasts for the number of airplane reservations to Italy suggest drops of 95.2 percent in June, 82.4 percent in July and 76.4 percent in August, compared to the same periods last year, according to Italy’s national tourism agency, ENIT.