Don't use a literal for TYPE_CHECKING (#7062)

Partly addresses #7061

stdlib/@python2/typing.pyi

@@ -304,7 +304,7 @@ class MutableMapping(Mapping[_KT, _VT], Generic[_KT, _VT]):
 
 Text = unicode
 
-TYPE_CHECKING: Literal[True]
+TYPE_CHECKING: bool
 
 class IO(Iterator[AnyStr], Generic[AnyStr]):
     # TODO detach

stdlib/typing.pyi

@@ -502,7 +502,7 @@ class MutableMapping(Mapping[_KT, _VT], Generic[_KT, _VT]):
 
 Text = str
 
-TYPE_CHECKING: _Literal[True]
+TYPE_CHECKING: bool
 
 # In stubs, the arguments of the IO class are marked as positional-only.
 # This differs from runtime, but better reflects the fact that in reality