debian, asn1tostruct.py: switch to python3

Make build work with python3 and drop the python2 dependency.

Related: OS#2819
Change-Id: Idb9d7a349ddb7619c784823f740648c7956bae5e
diff --git a/debian/control b/debian/control
index 7e760acb..d07ab67 100644
--- a/debian/control
+++ b/debian/control
@@ -14,7 +14,7 @@
                libosmocore-dev (>= 0.12.0),
                libosmo-netif-dev (>= 0.3.0),
                libosmo-sigtran-dev (>= 0.10.0),
-               python (>= 2.7)
+               python3
 Standards-Version: 3.9.8
 Vcs-Git: git://git.osmocom.org/osmo-iuh.git
 Vcs-Browser: https://git.osmocom.org/osmo-iuh/