Add TypeAlias for OrderedDict

[elazarg]

This is needed for NamedTuple support in mypy

[JukkaL]

Hmm, I don't think that this is required. If we want to use OrderedDict, it should be possible to use it via collections. I'd rather not have things in typing stubs that aren't present in the implementation.

[elazarg]

So add the same TypeAlias there, or use plain ordereddict? This is somewhat confusing.
What's the difference between OrderedDict and DefaultDict?

[JukkaL]

collections only seems to have OrderedDict, I can't see ordereddict. Type aliases don't help here -- you need to refer to the real class definition, not an alias. Aliases are resolved elsewhere.

[gvanrossum]

You no longer need this right?