Some dkimpy versions have a bug where it can't parse the keys generated
by its own key generator. That causes the dkimpy test to fail.
See https://bugs.launchpad.net/dkimpy/+bug/1978835 for more details.
This patch adds a workaround which detects the buggy version, and skip
the test if needed.
This patch adds support in the default hook for using dkimpy for DKIM
signing.
Unfortunately, dkimpy binaries have the same name as driusan/dkim's, so
we need to use --help to disambiguate. It's not pretty but it should
work, and is quite self contained.
Also, for the integration tests, we still need driusan/dkim because
dkimpy lacks the features needed. Specifically, dkimpy's dkimverify
can't be made to use custom DNS, or override the TXT values in any way,
so we can't verify that the generated signature is reasonable.
Thanks to ne9z@github for suggesting this change and providing an
alternative patch in https://github.com/albertito/chasquid/pull/19.
