Work around API change in Astropy 5

In Astropy 4, `z_at_value` returns a float, but in Astropy 5,
returns a quantity with dimensionless redshift units. Work around
this by making sure that its return value is a Quantity before we
convert it to a float.

Note: remove this workaround once we drop support for astropy < 5.
17 jobs for master in 82 minutes and 34 seconds (queued for 3 seconds)
Status Name Job ID Coverage
  Deps
passed requirements #1574619

00:00:10

 
  Dist
passed dependencies/python3.7 #1574623

00:02:03

passed dependencies/python3.8 #1574624

00:00:09

passed dependencies/python3.9 #1574625

00:02:04

passed sdist #1574620

00:00:29

passed wheel/macosx #1574622
macos_sierra

00:01:11

passed wheel/manylinux #1574621

00:00:44

 
  Test
passed docs #1574626

00:09:19

passed lint #1574632

00:00:48

passed test/coverage #1574631

01:19:36

81.78%
passed test/dev-dependencies #1574630

00:19:48

passed test/python3.7 #1574627

00:12:43

failed test/python3.8 #1574628

00:20:15

passed test/python3.9 #1574629

00:12:45

manual tests/review #1574633
allowed to fail manual
 
  Deploy
passed pages #1574634

00:00:39

passed pages:deploy

00:00:01

 
Name Stage Failure
failed
test/python3.8 Test
18.85s call     postprocess/tests/test_crossmatch.py::test_crossmatch_cartesian_gaussian_distribution[0-0]
=========================== short test summary info ============================
FAILED ../../usr/local/lib/python3.8/site-packages/ligo/skymap/postprocess/crossmatch.py::skymap.postprocess.crossmatch.crossmatch
=========== 1 failed, 966 passed, 69 warnings in 1194.33s (0:19:54) ============
Running after_script
Running after script...
$ cd "${CI_PROJECT_DIR}"
$ rm -rf "${WORKING_DIRECTORY}"
Cleaning up file based variables
ERROR: Job failed: exit code 1