Support more than just string literals with TypedDict indexing #7867
Description
from mypy_extensions import TypedDict
T = TypedDict('T', {'a': float, 'b': float, 'c': float})
foo: T = {'a': 0.0, 'b': 0.0, 'c': 0.0}
for key in ('a', 'b', 'c'):
foo[key] = 1.0
mypy currently (0.730) complains about that last line, saying error: TypedDict key must be a string literal; expected one of ('a', 'b', 'c'). It's understandable, but limiting - it'd be great if mypy could recognise that key is in fact one of the valid keys for T.