TypedDict item insertion support

• Are you reporting a bug, or opening a feature request?
  Feature request
• Please insert below the code you are checking with mypy,
  or a mock-up repro if the source is private. We would appreciate
  if you try to simplify your case to a minimal repro.
  Currently I can't insert items into TypedDict even after explicit check for a defined key:

from mypy_extensions import TypedDict

GroupDataDict = TypedDict(
    'GroupDataDict', {"children": str,
                      "vars": str,
                      "hosts": str}, total=False
)

test_group: GroupDataDict = {}
key = "children"
if key in ("children", "vars"):
    test_group[key] = "a"

if key not in ["children", "vars"]:
    raise ValueError("")
test_group[key] = "b"

print(test_group)

• What is the actual behavior/output?

-> % mypy typeddict_insertion.py
typeddict_insertion.py:12: error: TypedDict key must be a string literal; expected one of ('children', 'vars', 'hosts')
typeddict_insertion.py:16: error: TypedDict key must be a string literal; expected one of ('children', 'vars', 'hosts')

• What is the behavior/output you expect?
  No error
• What are the versions of mypy and Python you are using?
  mypy 0.620 + Python 3.7
• Do you see the same issue after installing mypy from Git master?
  I haven't tried
• What are the mypy flags you are using? (For example --strict-optional)

[mypy]
python_version = 3.7

check_untyped_defs = True
disallow_any_generics = True
disallow_untyped_calls = True
ignore_errors = False
ignore_missing_imports = True
strict_optional = True
warn_unused_ignores = True
warn_redundant_casts = True
warn_unused_configs = True

Commencez par exécuter l’exemple TypedDict fourni avec la version actuelle de mypy et examinez comment les expressions de clés TypedDict sont vérifiées. Le travail est terminé lorsque des vérifications explicites qui restreignent une clé aux noms TypedDict valides autorisent les affectations présentées sans erreur, tandis que les clés invalides restent rejetées ; ajoutez ou mettez à jour des tests ciblés pour les deux cas.