Recent change to MROs of built-in types caused Python 2/3 inconsistency

[JukkaL]

#2602 caused this to behave differently in mypy Python 2 and 3 modes:

reveal_type(['a', {'b'}])

In Python 2 mode it shows this:

Revealed type is 'builtins.list[typing.Sized*]'

In Python 3 mode it shows this:

Revealed type is 'builtins.list[typing.Collection*[builtins.str]]'

Before the PR Python 2 behavior was closer to Python 3:

Revealed type is 'builtins.list[typing.Iterable*[builtins.str]]'

This change causes regressions for mypy users, since the old type was more precise. More generally, any change in the MROs of built-in types has a significant chance of generating regressions and should be tested carefully.

Not sure what's the best way to move forward. I'd prefer getting Python 2 and Python 3 to be more similar if possible. Maybe we don't need to include Sized explicitly in the MRO since it's a protocol? Again, this could have unforeseen results.

cc @gvanrossum

[srittau]

I'm generally in favor of not including protocols in base classes, although in this case it makes some sense, since Sequence is part of the basic type hierarchy. For now I suggest to revert this change until mypy 0.650 is released and discuss this afterwards (and live with the results of that discussion).

[gvanrossum]

Reopening because I want a different fix (see #2658).