Bug#1077610: dnsviz: Drop m2crypto dependency

[Bastian Germann]

Source: dnsviz
Version: 0.9.4-1

Please drop the (Build-)Depends on python3-m2crypto, which is not strictly 
needed for dnsviz.
m2crypto should be removed from Debian because it is explicitly in maintenance 
mode.
dnsviz is one of the last few reverse dependencies.