bpo-42103: Improve validation of Plist files. (GH-22882)

bpo-42103: Improve validation of Plist files. (GH-22882)

* Prevent some possible DoS attacks via providing invalid Plist files
  with extremely large number of objects or collection sizes.
* Raise InvalidFileException for too large bytes and string size instead of returning garbage.
* Raise InvalidFileException instead of ValueError for specific invalid datetime (NaN).
* Raise InvalidFileException instead of TypeError for non-hashable dict keys.
* Add more tests for invalid Plist files.