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

[kpdev42]

Summary

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

[thopre]

I like the simplicity of it compared to my solution, but I'm a bit bothered by plistformat2 being in fact the same format. But it is simpler and less ugly than what I did. @tnfchris what is your opinion?

@kpdev42 Are you still using LNT in Python 2 mode? If not maybe we should just drop Python 2 support (https://reviews.llvm.org/D69056)

[thopre]

Alright, let's keep this version. Thanks!

lnt/formats/PlistFormat2.py

+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

 """
 Utilities for converting to LNT's test format.
 
 LNT formats are described by dictionaries with 'name', 'read', and 'write'
 fields. Only the 'name' field is required. The 'read' field should be a
 callable taking a path_or_file object, the 'write' function should be a
 callable taking a Python object to write, and the path_or_file to write to.
 """
 
 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]
 formats_by_name = dict((f['name'], f) for f in formats)  # type: Dict[str, Dict]
 format_names = list(formats_by_name.keys())  # type: List[str]
 
 
 def get_format(name):