pytests: check minimum required gnutls version

Add a message to make extra requirements clear instead of throwing
a compilation error.

Closes #457
27 jobs from pytests-check-gnutls in 7 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
canceled #254633
docker linux amd64
archive

00:00:07

canceled #254634
docker linux amd64
build

00:00:07

canceled #254635
docker linux amd64
build-asan

00:00:07

canceled #254636
docker
kres-gen

00:00:07

 
  Test
canceled #254637
docker delayed
build:darwin

canceled #254638
docker linux amd64
deckard

canceled #254639
docker delayed
doc

canceled #254640
dind
docker

canceled #254641
docker delayed
lint:other

canceled #254642
docker linux amd64 delayed
lint:pedantic

canceled #254643
docker linux amd64
lint:scan-build

canceled #254644
docker linux amd64
lint:tidy

canceled #254645
privileged
pkg:epel-7

canceled #254646
privileged
pkg:fedora-29

canceled #254647
docker linux amd64
respdiff:basic

canceled #254648
docker linux amd64 delayed
test:valgrind

 
  Extended
canceled #254650
respdiff
fwd-tls6-kresd.udp6

canceled #254651
respdiff
fwd-udp6-kresd.udp6

canceled #254655
respdiff
fwd-udp6-unbound.tcp6

canceled #254656
respdiff
fwd-udp6-unbound.tls6

canceled #254654
respdiff
fwd-udp6-unbound.udp6

canceled #254653
respdiff
iter.tls6

canceled #254652
respdiff
iter.udp6

canceled #254649
docker linux amd64
pytests

canceled #254657
respdiff
resperf:fwd-tls6.udp-asan

canceled #254658
respdiff
resperf:fwd-udp6.udp-asan

canceled #254659
respdiff
resperf:iter.udp-asan