sanitize mirror URLs to prevent path segments from being removed

sanitize mirror URLs to prevent path segments from being removed

urllib.parse.urljoin() will strip off the last path segment before joining
if that last path segment does not end with /.  That's a "feature" to make
it easy to replace file names.  Here it was stripping off the essential
'fdroid' segment, making URLs like:

https://foo.com/repo

when they should be

https://foo.com/fdroid/repo