Move Sized earlier in the bases of Sequence

[gvanrossum]

See rominf/ordered-set-stubs#1:

class OrderedSet(MutableSet[T], Sequence[T]): ...

works in Python 3, but not in Python 2 -- this fixes that.

(I haven't run the tests yet -- if they show problems with this I may just withdraw this.)

[JelleZijlstra]

Seems fine, although this feels a bit unprincipled—how do we know this won't break some other way of subclassing the typing classes?

[gvanrossum]

Indeed, we don't know. Though then that other way would break with Python 3.

FWIW I looked at the source of typing.py, and there it's different again: The Python 2 source derives Sequence from either (Sized, Reversible, Container), or (Sized, Iterable, Container), while the Python 3 source allows those two or (Reversible, Collection), where Collection derives from (Sized, Iterable, Container). So the relative order of Sized and Reversible is not fixed, but I didn't change that in this PR -- I'm only making Sized come before Iterable consistently, which matches that.

(Also note that calculating the MRO is not black magic -- it finds a unique solution that preserves the relative order of base classes and satisfies some other constraint that makes it unique.)