Build-Depends on nettle-dev >=3.1

Upstream version 2.79 has this in the changelog:

+ Tidy up Crypto code, removing workarounds for ancient
+ versions of libnettle. We now require libnettle 3.

Not sure this is backportable to Trusty's nettle on 2.7.1-1ubuntu0.2 without taking back the upstream changes.

To get you correctly, you report that the build dependency currently being
 nettle-dev (>=2.4-3)
should be like
 nettle-dev (>=3.1-1)
right?

I agree, when 2.79 was merged which implies these dependencies that should have been bumped.

Since it "only" affects backports to very old releases as we have
  Xenial: 3.2-1ubuntu0.16.04.1
  Debian-Stable: 3.3-1
I'll mark importance low for now.

Thank you for taking the time to report this bug and helping to make Ubuntu better.

This bug is present in Debian too, and Ubuntu currently doesn't make any changes over the Debian package. So this bug would be best fixed directly in Debian, and then Ubuntu will pick up the fix automatically.

Would you mind filing a bug with Debian please?
I think given the low severity it is fine to fix it just there and we pick it up automatically.
If you do so I'd be glad if you could report back the generated bug number to track the handling there.