Fixed plistlib API usage for both python 2.7 and 3.x

Reverted plistlib API for python 2.7
Related to: https://reviews.llvm.org/D109235 and https://reviews.llvm.org/D112136

Reviewed By: thopre

Differential Revision: https://reviews.llvm.org/D112202

Author: Thomas Preud'homme

lnt/formats/PlistFormat2.py

@@ -0,0 +1,17 @@
+import plistlib
+
+
+def _matches_format(path_or_file):
+    try:
+        plistlib.readPlist(path_or_file)
+        return True
+    except Exception:
+        return False
+
+
+format = {
+    'name': 'plist',
+    'predicate': _matches_format,
+    'read': plistlib.readPlist,
+    'write': plistlib.writePlist,
+}

lnt/formats/__init__.py

@@ -9,7 +9,11 @@
 
 from __future__ import absolute_import
 from typing import List, Dict
-from .PlistFormat import format as plist
+try:
+    from plistlib import readPlist
+    from .PlistFormat2 import format as plist  # for Python 2
+except ImportError:
+    from .PlistFormat import format as plist  # for Python 3
 from .JSONFormat import format as json
 
 formats = [plist, json]  # type: List[Dict]