setup: Fix for py3

py2-only syntax introduced in acf7e91d (setup/runcmd: Properly report
errors, if running command is missing):

      File "setup.py", line 168
        except Exception, e:
                        ^
    SyntaxError: invalid syntax

setup.py

@@ -172,7 +172,7 @@ def register_as_entrypoint(func, entryname, groupname, distname):
 def runcmd(argv):
     try:
         process = Popen(argv, stdout=PIPE)
-    except Exception, e:
+    except Exception as e:
         raise RuntimeError("%s: %s" % (argv, e))
 
     output, _err = process.communicate()