Skip test if using very old version of pytest

This test requires the `pytest.approx` version, which was added in
pytest 3.0.0.
12 jobs for pytest-version in 45 minutes and 40 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Level0
passed #17495
level0:lal

00:10:21

 
  Level1
passed #17496
level1:lalframe

00:03:02

passed #17497
level1:lalmetaio

00:01:05

passed #17498
level1:lalsimulation

00:08:14

passed #17499
level1:lalxml

00:01:50

 
  Level2
passed #17500
level2:lalburst

00:02:36

passed #17501
level2:lalinspiral

00:07:23

passed #17502
level2:lalpulsar

00:04:02

passed #17503
level2:lalstochastic

00:02:19

 
  Level3
passed #17504
level3:laldetchar

00:02:42

passed #17505
level3:lalinference

00:09:50

 
  Level4
passed #17506
level4:lalapps

00:09:47