input: Write a python function to block sites using a proxy URL.
  assert block_sites('http://example.com/proxy') == ['blockedsite1.com', 'blockedsite2.com']
  assert block_sites('http://example.com/proxy2') == ['blockedsite3.com', 'blockedsite4.com']
  assert block_sites('http://example.com/proxy3') == ['blockedsite5.com', 'blockedsite6.com']
output: def block_sites(proxy_url):
      blocked_sites_mapping = {
          'http://example.com/proxy': ['blockedsite1.com', 'blockedsite2.com'],
          'http://example.com/proxy2': ['blockedsite3.com', 'blockedsite4.com'],
          'http://example.com/proxy3': ['blockedsite5.com', 'blockedsite6.com'],
      }
      return blocked_sites_mapping.get(proxy_url, [])