Weaken cryptography requirement to >=42

[cjwatson]

dnspython's dnssec extra only needs cryptography 42, not 42.0.7, so it seems unnecessary to depend on a newer patch release. (I noticed this because Debian currently has 42.0.5, and the changes between 42.0.5 and 42.0.7 seem insignificant for DNSSEC validation.)