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

Start by running the supplied TypedDict example with the current mypy version and inspect how TypedDict key expressions are checked. The work is done when explicit checks that restrict a key to valid TypedDict names allow the shown assignments without errors, while invalid keys remain rejected; add or update focused tests for both cases.