I am not sure, if this is the right place to report it, and if the problem is not due to my setup, but something is strange.
I freshly installed Mercurial 2.5 and after testing it worked, added the keyring extension, which caused a failure when attempting to clone with an error complaining about configparser module not found. I am on Python 2.7.3, and the module is named ConfigParser, but the keyring extension implements as py27compat module that should deal with it. However, due to Mercurials demandimport.py module this does not seem to work properly here.
Putting 'configparser' into the ignore list of demandimport.py resolved the issue. I have no idea, if this is the correct thing to do, though.