Fix return type of KWallet get_credential()
Fix return type of KWallet get_credential()

Docs:
get_credential(service, username): Return a credential object stored in
the active keyring. This object contains at least username and password
attributes for the specified service, where the returned username may be
different from the argument.

Prior to this commit, get_credential() returned the password as a string,
without wrapping the credential into a SimpleCredential object.

jaraco/keyring