Support for default types in TypedDict

This problem arose from trying to type the environ argument for a WSGI application. There are specific keys of the environ dict that have non-str types, and then all the other keys are HTTP_ variables (or server-defined variables, which can be ignored for this purpose) with type str (or Text, depending on the version of Python).

I would like to get stronger typing on environ than Dict[str, Any], but there isn't a way to do that with TypedDict currently because the set of possible keys is unbounded (the client can send any HTTP header).

Ideally, I'd like to be able to do something like:

EnvironDict = TypedDict("EnvironDict", {
    "wsgi.version": Tuple[int, int],
    "wsgi.input": IO[str],
# ...
}, default_type=str, total=False)

and have this mean that any key is allowed in the dictionary and all unknown keys get a value type of str.

The issue describes extending TypedDict with a default value type for otherwise unknown keys, using the WSGI environ example as motivation. Start by reviewing mypy’s existing TypedDict support and its tests; done means the proposed syntax is defined, arbitrary keys receive the default type, explicit keys retain their types, and the behavior is covered by tests.