Fix handling of file inputs on Windows; passing them to urllib.urlopen()

caused the drive letter to cause urlopen() to think it was an unrecognized
URL scheme.  This only passes system ids to urlopen() if the file does not
exist.  It works on Windows & Unix.

It should work everywhere else as well.

Lib/xml/sax/saxutils.py

@@ -198,14 +198,16 @@ def prepare_input_source(source, base = ""):
         source = xmlreader.InputSource(source)
         source.setByteStream(f)
 
-    if source.getByteStream() == None:
+    if source.getByteStream() is None:
         sysid = source.getSystemId()
-        if urlparse.urlparse(sysid)[0] == '':
+        if os.path.isfile(sysid):
             basehead = os.path.split(os.path.normpath(base))[0]
             source.setSystemId(os.path.join(basehead, sysid))
+            f = open(sysid, "rb")
         else:
             source.setSystemId(urlparse.urljoin(base, sysid))
+            f = urllib.urlopen(source.getSystemId())
             
-        source.setByteStream(urllib.urlopen(source.getSystemId()))
+        source.setByteStream(f)
         
     return source