Fix type hints pickling in python 3.6

[lhoestq]

Type hints can't be properly pickled in python 3.6. This was causing errors the run_mlm.py script from transformers with python 3.6

However Cloupickle proposed a fix to make it work anyway.
The idea is just to implement the pickling/unpickling of parameterized type hints. There is one detail though: since in python 3.6 we can't use isinstance on type hints, then we can't use pickle saving functions registry directly. Therefore we just wrap the save_global method of the Pickler.

This should fix huggingface/transformers#8212 for python 3.6 and make run_mlm.py support python 3.6

cc @sgugger