Use new plistlib API

[thopre]

Summary

Old plistlib API (e.g. readPlist and writePlist methods) has been
dropped in Python 3.9. Update LNT to use the new Python 3 API.

[cmatthews]

LGTM

lnt/formats/PlistFormat.py

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