Fix Python2 support in PlistFormat.py

[slydiman]

Looks good.
Probably https://reviews.llvm.org/D112202 is little bit faster on python 2.7.

[thopre]

In D112185#3077090, @slydiman wrote:

Looks good.
Probably https://reviews.llvm.org/D112202 is little bit faster on python 2.7.

Yeah and it looks nicer. Ok, abandonning this one.

lnt/formats/PlistFormat.py

 import plistlib
 
 
-def _matches_format(path_or_file):
+def _load_format(path_or_file):
     try:
         if isinstance(path_or_file, str):
             with open(path_or_file, 'rb') as fp:
-                plistlib.load(fp)
+                return plistlib.load(fp)
         else:
-            plistlib.load(path_or_file)
+            return plistlib.load(path_or_file)
+    except AttributeError:
+        return plistlib.readPlist(path_or_file)
+
+
+def _matches_format(path_or_file):
+    try:
+        _load_format(path_or_file)
         return True
     except Exception:
         return False
 
 
-def _load_format(path_or_file):
-    if isinstance(path_or_file, str):
-        with open(path_or_file, 'rb') as fp:
-            return plistlib.load(fp)
-    else:
-        return plistlib.load(path_or_file)
+def _dump_format(obj, fp):
+    try:
+        plistlib.dump(obj, fp)
+    except AttributeError:
+        plistlib.writePlist(obj, fp)
 
 
 format = {
     'name': 'plist',
     'predicate': _matches_format,
     'read': _load_format,
-    'write': plistlib.dump,
+    'write': _dump_format,
 }