Fix "specific target" detection in Python 3 (#12906)

The output of subprocess.check_output is a byte string in Python 3. This causes dictionary lookup to fail later on.

A try-except around this function prevented this error from being noticed. Removed this so that more errors can propagate out.

lib/spack/llnl/util/cpu/detect.py

@@ -7,7 +7,7 @@
 import platform
 import re
 import subprocess
-import sys
+import warnings
 
 import six
 
@@ -76,11 +76,8 @@ def proc_cpuinfo():
 
 
 def check_output(args):
-    if sys.version_info[:2] == (2, 6):
-        return subprocess.run(
-            args, check=True, stdout=subprocess.PIPE).stdout  # nopyqver
-    else:
-        return subprocess.check_output(args)  # nopyqver
+    output = subprocess.Popen(args, stdout=subprocess.PIPE).communicate()[0]
+    return six.text_type(output.decode('utf-8'))
 
 
 @info_dict(operating_system='Darwin')
@@ -126,8 +123,8 @@ def raw_info_dictionary():
     for factory in info_factory[platform.system()]:
         try:
             info = factory()
-        except Exception:
-            pass
+        except Exception as e:
+            warnings.warn(str(e))
 
         if info:
             break