[Python-ideas] Add new `Symbol` type

[Flavio Curella]

@Nathaniel Smith:

> I think the name "symbol" here is pretty confusing. It comes originally
from Lisp
> The thing you're talking about is what Python devs call a "sentinel"
object.

 Thank you for clarifying. I don't know much about Lisp, and I definitely
appreciate the historical context that you provided :)

I will refer to the new proposed type as `sentinel` from now on.

@Michael Foord

> unittest.mock includes a sentinel object, with nice repr.

Thank you! I didn't think of looking in the mock library.

This is one step closer, but it still have a few considerations:
1. It doesn't follow the same behaviour as the JS spec. But honestly, we
don't have to.
2. It's kinda weird to have to import `unittest.<something>` in code that's
not tests. But I think it's just because I'm not used to see it.

@Giampaolo Rodola'
> Historically this has always been achieved by using [...] which does its
job just fine.

The main issue with that approach is that you won't get a nice repr

> adding more complexity to the language for no practical benefit

I'm not following. The Python language won't be modified. I'm proposing
adding the new type _purely_ for practical benefit.

I think this thread can be resolved as 'used unittest.mock.sentinel'. It
doesn't have 'global sentinels', but I'm not convinced they are actually
necessary, since `mock.sentinel` objects with the same name compare as
equal. Thanks to Nathaniel, I now understand that JS has global symbols for
historical reasons that we don't have, and I'm not convinced of their
usefulness.

Thank you everybody for you valuable feedback! I really appreciate your
time helping me thinking this through :)
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