Serializer understand `Optional[...]`, but not `...|None`

[Carreau]

Not critical if we use Optional[...] in the code base but it would be nice to support.

In [1]: from papyri.gen import Node
   ...: from typing import Optional
   ...: class Foo(Node):
   ...:     a : Optional[int]
   ...:     b : int | None
   ...: Foo(1, 2).to_dict()
Out[1]: {'type': 'Foo', 'a': 1, 'b': None}
#                                    ^ should be 2