Accept None as a key in pure python module

[TIGirardi]

Currently, the pure python module (map.py) uses None as a marker for child nodes.

However, None is hashable and it is possible to be used as a key both in the builtin dict and in the C Map implementation.

The python version actually accepts setting None, but every subsequent operation that reaches the key mistakes it for a node marker and tries to operate on the value.

[1st1]

I'd call it NULL, the module isn't imported directly anyways. Also I think you can just use void since this is just a marker that users never see.

[jab]

+1. Note, void is hashable (object implements __hash__ without passing it down to subclasses), but that doesn’t matter if this is just used as a sentinel object anyway?

>>> foo = object()
>>> hash(foo)
9007199254740991

[TIGirardi]

I guess I'm being a little paranoid, but I would prefer an immediate error, which void wouldn't do. This is the current behavior:

>>> from immutables.map import Map
>>> m = Map().set(None, 1)
# do a bunch of stuff
>>> None in m
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "$prefix/lib/site-packages/immutables/map.py", line 592, in __contains__
    self.__root.find(0, map_hash(key), key)
  File "$prefix/lib/site-packages/immutables/map.py", line 157, in find
    return val_or_node.find(shift + 5, hash, key)
AttributeError: 'int' object has no attribute 'find'

With the patch (if somehow the user uses _NULL as key) it raises a TypeError immediately.

Whatever you decide, I can change with a button click.

[1st1]

Alright, I merged as is. Thanks so much for help! I'll release tomorrow.

[TIGirardi]

Thank you!