Support TypedDicts with missing keys (total=False) (#3558)

* Basic support for TypedDicts with missing keys (total=False)

Only the functional syntax is supported.

* Support get(key, {}) and fix construction of partial typed dict

* Fix subtyping of non-total typed dicts

* Fix join with non-total typed dict

* Fix meet with non-total typed dicts

* Add serialization test case

* Support TypedDict total keyword argument with class syntax

* Attempt to fix Python 3.3

* Add minimal runtime `total` support to mypy_extensions

There is no support for introspection of `total` yet.

* Fix tests on pre-3.6 Python and improve introspection

Make TypedDict `total` introspectable.

* Fix lint

* Fix problems caused by merge

* Allow td['key'] even if td is not total

* Fix lint

* Add test case

* Address review feedback

* Update comment

mypy_extensions.py

diff --git a/mypy_extensions.py b/mypy_extensions.py
index 82eea32..c711e00 100644
--- a/mypy_extensions.py
+++ b/mypy_extensions.py
@@ -30,16 +30,18 @@
 
 
 def _typeddict_new(cls, _typename, _fields=None, **kwargs):
+    total = kwargs.pop('total', True)
     if _fields is None:
         _fields = kwargs
     elif kwargs:
         raise TypeError("TypedDict takes either a dict or keyword arguments,"
                         " but not both")
-    return _TypedDictMeta(_typename, (), {'__annotations__': dict(_fields)})
+    return _TypedDictMeta(_typename, (), {'__annotations__': dict(_fields),
+                                          '__total__': total})
 
 
 class _TypedDictMeta(type):
-    def __new__(cls, name, bases, ns):
+    def __new__(cls, name, bases, ns, total=True):
         # Create new typed dict class object.
         # This method is called directly when TypedDict is subclassed,
         # or via _typeddict_new when TypedDict is instantiated. This way
@@ -59,6 +61,8 @@
         for base in bases:
             anns.update(base.__dict__.get('__annotations__', {}))
         tp_dict.__annotations__ = anns
+        if not hasattr(tp_dict, '__total__'):
+            tp_dict.__total__ = total
         return tp_dict
 
     __instancecheck__ = __subclasscheck__ = _check_fails