Small cleanup to ring serialization

Instead of catching TypeError and retrying without mtime, let's just
check and see if mtime is an allowed arg and decide based on
that. This way, if anything else raises TypeError we'll know about it.

Change-Id: Ib394d8176e2bc4bd53ace019ce5816369223c273

swift/common/ring/ring.py

@@ -15,6 +15,7 @@
 
 import array
 import cPickle as pickle
+import inspect
 from collections import defaultdict
 from gzip import GzipFile
 from os.path import getmtime
@@ -112,10 +113,10 @@ class RingData(object):
         # This only works on Python 2.7; on 2.6, we always get the
         # current time in the gzip output.
         tempf = NamedTemporaryFile(dir=".", prefix=filename, delete=False)
-        try:
+        if 'mtime' in inspect.getargspec(GzipFile.__init__).args:
             gz_file = GzipFile(filename, mode='wb', fileobj=tempf,
                                mtime=mtime)
-        except TypeError:
+        else:
             gz_file = GzipFile(filename, mode='wb', fileobj=tempf)
         self.serialize_v1(gz_file)
         gz_file.close()