Prepend bases on first subscription

[ilevkivskyi]

Fixes#385
Fixes#384

In previous versions of typing, __bases__ on subscription where prepend with __origin__, so that MRO of, e.g., Mapping[int, T][str] was:

[Mapping[int, str], Mapping[int, T], Mapping, collections.abc.Mapping, ...]

This was removed for several reasons. However, now it looks like complete removal of such prepending was too radical, since now e.g. issublcass(Node[int], Node) returns False. I believe such runtime check should be equivalent to issublcass(Node, Node), i.e., True.

Here I propose to fix this by only prepending __bases__ on first subscription, so that, e.g, Mapping[int, T][str].__mro__ will be:

[Mapping[int, str], Mapping, collections.abc.Mapping, ...]

This PR also fixes an ambiguity when using typing ABCs with singledispatch().

[ilevkivskyi]

This will also allow to simplify #375 to only adding tests. So that I would like to merge this now.
@gvanrossum I am mentioning you just in case, so that this will not pass unnoticed.