[3.6] bpo-42103: Improve validation of Plist files. (GH-22882) (GH-23118)

* 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..
(cherry picked from commit 34637a0ce21e7261b952fbd9d006474cc29b681f)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>