picard-plugins-aux: new location for metaflac plugin
python-keyring: +optdepend python-secretstorage

Tests fail when python-secretstorage is installed when building the
package. However, this is independent of secretstorage being listed as
optedend or not.

Solution is a patch of secretstorage (or version > 1.1.0),
mentioned in
https://bitbucket.org/kang/python-keyring-lib/issue/132/install-tests-already-try-to-initialize