Compatibility improvements for GNOME Keyring backend

- Use the same attributes (``username`` / ``service``) as the SecretService
  backend uses, allow searching for old ones for compatibility.
- Also set ``application`` attribute.
- Correctly handle all types of errors, not only ``CANCELLED`` and ``NO_MATCH``.
Secret Service: use a better label for passwords, the same as GNOME Keyring backend uses
GnomeKeyring: Avoid printing warnings to stderr when GnomeKeyring is not available
Replaced release.py with a reference to jaraco.packaging.
Added tag 1.6 for changeset fbce773ad6c1
Bumped to 1.7 in preparation for next release.
Merge with 1.6

--HG--
branch : prioritized backends