restore extra check for subprocess32 import

Author: mr-c

cwltool/singularity.py

@@ -12,7 +12,7 @@
 from .pathmapper import PathMapper, ensure_writable
 from .process import (UnsupportedRequirement)
 from .utils import docker_windows_path_adjust
-if sys.version_info[0] < 3:
+if os.name == 'posix' and sys.version_info[0] < 3:
     import subprocess32 as subprocess
 else:
     import subprocess  # type: ignore