ATHENS — Just when you thought it was safe to climb up the Acropolis to take a selfie, the likelihood is growing that the ancient site could be closed off to tourists during what are normally the busiest times to visit: midday to early evening. Greece is not only grappling with the same searing temperatures that are gripping Italy and other holiday hotspots across southern Europe, but is also battling major wildfires on the perimeter of the capital.