Change download URL

It looks like the following Amazon domains no longer work:
- `z2-ec2.images-amazon.com`: "invalid hostname" error
- `s3.cn-north-1.amazonaws.com.cn`: "access denied" error

After some experimentation it was determined that covers are now being
served under the `ec2.images-amazon.com` domain.

This commit adds the new URL and removes the non-working ones.

__init__.py

@@ -36,8 +36,10 @@ def download_cover(self, log, result_queue, abort,
 
 def get_cover_urls(log, title, authors, identifiers, timeout):
     sources = frozenset([
-        'http://z2-ec2.images-amazon.com/images/P/{0}.01.MAIN._SCRM_.jpg',
-        'https://s3.cn-north-1.amazonaws.com.cn/sitbweb-cn/content/{0}/images/cover.jpg',
+        'https://ec2.images-amazon.com/images/P/{0}.01.MAIN._SCRM_.jpg',
+        # No longer seem to work as of 2024-02-04:
+        #'http://z2-ec2.images-amazon.com/images/P/{0}.01.MAIN._SCRM_.jpg',
+        #'https://s3.cn-north-1.amazonaws.com.cn/sitbweb-cn/content/{0}/images/cover.jpg',
     ])
     urls = set()
     asins = set()