ARC Sign / Verify in Postfix

Question

I've upgraded my Postfix mail system from opendkim to dkimpy in Debian Stretch using Backports, because I'm trying to implement ARC signing and verification. I was able to get dkimpy working in so far that DKIM verification was done on inbound mail, but couldn't get DKIM signing to work and found little to no information / documentation on ARC in dkimpy. Any help would be appreciated. My config below.

# /etc/postfix/master.cf

smtp       inet  n       -       y       -       -       smtpd
    -o smtpd_milters=inet:localhost:8892
    -o milter_macro_daemon_name=VERIFYING

submission inet  n       -       y       -       -       smtpd
        -o syslog_name=postfix/submission
        -o smtpd_tls_security_level=encrypt
        -o smtpd_sasl_auth_enable=yes
        -o smtpd_milters=inet:localhost:8892
        -o non_smtpd_milters=inet:localhost:8892
        -o milter_macro_daemon_name=ORIGINATING


# /etc/dkimpy-milter.conf
Domain                  *
KeyFile                 /etc/pki/dkim/alpha.private
Selector                alpha
Canonicalization        relaxed/simple
Mode                    sv
Socket                  inet:8892@localhost
PidFile                 /var/run/dkimpy-milter/dkimpy-milter.pid
UserID                  dkimpy-milter
MacroList               daemon_name|ORIGINATING
MacroListVerify         daemon_name|VERIFYING