Merge pull request #4456 from jcftang/devel

This oneliner change allows apt-key to inherit the http_proxy variable.

library/packaging/apt_repository

@@ -304,7 +304,7 @@ class UbuntuSourcesList(SourcesList):
 
             if self.add_ppa_signing_keys_callback is not None:
                 info = self._get_ppa_info(ppa_owner, ppa_name)
-                command = ['apt-key', 'adv', '--recv-keys', '--keyserver', 'keyserver.ubuntu.com', info['signing_key_fingerprint']]
+                command = ['apt-key', 'adv', '--recv-keys', '--keyserver', 'hkp://keyserver.ubuntu.com:80', info['signing_key_fingerprint']]
                 self.add_ppa_signing_keys_callback(command)
 
             file = file or self._suggest_filename('%s_%s' % (line, distro.codename))