duplicity public-key-only incompatible with gnupg 2.0.11

[Anthem]

Applies to duplicity 0.5.17 and 0.6.2 under Linux.

Scenario:
duplicity --encrypt-key 1E05E475 --sign-key 93E42BEB --archive-dir /duplicity-archive /backup file:///duplicity-repository/
Only the public key part of the encryption key is present, both public and private keys of a separate signature key are present.

While the initial full backup will work, the incremental backup will fail with a gpg error saying it cannot decrypt the manifest. The --archive-dir switch does not seem to have any effect.

Solution 1: Either downgrade to GnuPG 1.4.9, or

Solution 2: modify the following line in collections.py:
            if "secret key not available" in message.args[0]:
to
            if "No secret key" in message.args[0]:

(The construction of an appropriate else-if-construct is left as an exercise to someone with python programming experience :)