Fix unit test skip condition

I am using `pkg_resources.require()` to test for a sufficiently new
version of Pytest. This can raise `pkg_resources.VersionConflict`.
Then I actually import pytest, which could raise an `ImportError`
(well, really, that's impossible, if the preceding
`pkg_resources.require()` call returned without raising an exception.
9 jobs for fix-unittest-skip in 133 minutes and 10 seconds (queued for 1 second)
Status Job ID Name Coverage
  Nightly
passed #18941
nightly:mpi:jessie

00:43:52

passed #18942
nightly:openmp:jessie

00:47:09

passed #18943
nightly:python3:stretch

00:43:57

failed #18940
allowed to fail
nightly:top-level:buster

00:23:57

passed #18936
nightly:top-level:el7

00:45:16

passed #18937
nightly:top-level:stretch

00:44:52

passed #18970
nightly:top-level:trusty

00:31:59

passed #18939
nightly:top-level:xenial

00:35:39

failed #18938
nightly:top-level:trusty

00:27:11

 
Name Stage Failure
failed
nightly:top-level:buster Nightly There is an unknown failure, please try again
Makefile:403: recipe for target 'check-recursive' failed
make[2]: Leaving directory '/builds/leo-singer/lalsuite/lalsuite-6.48.1/_build/sub'
make[2]: *** [check-recursive] Error 1
Makefile:677: recipe for target 'check' failed
make[1]: Leaving directory '/builds/leo-singer/lalsuite/lalsuite-6.48.1/_build/sub'
make[1]: *** [check] Error 2
Makefile:594: recipe for target 'distcheck' failed
make: *** [distcheck] Error 1
ERROR: Job failed: exit code 1