fix singularity 3 test (#1353)

Author: mr-c

tests/util.py

@@ -62,14 +62,14 @@ def get_data(filename: str) -> str:
 )
 
 needs_singularity_2_6 = pytest.mark.skipif(
-    not (distutils.spawn.find_executable("singularity") and is_version_2_6()),
+    not bool(distutils.spawn.find_executable("singularity") and is_version_2_6()),
     reason="Requires that version 2.6.x of singularity executable version is on the system path.",
 )
 
 needs_singularity_3_or_newer = pytest.mark.skipif(
-    not (distutils.spawn.find_executable("singularity") and is_version_3_or_newer),
-    reason="Requires that version 2.6.x of singularity executable version is on the system path.",
-)
+     (not bool(distutils.spawn.find_executable("singularity"))) or (not is_version_3_or_newer()),
+     reason="Requires that version 3.x of singularity executable version is on the system path.",
+ )
 
 
 windows_needs_docker = pytest.mark.skipif(