Add interning of unicode strings by copying the functionality from
stringobject.c.

Intern "True" and "False" in bool_repr() again as it was in the
8bit string era.
diff --git a/Objects/boolobject.c b/Objects/boolobject.c
index 0a9f958..b0170f6 100644
--- a/Objects/boolobject.c
+++ b/Objects/boolobject.c
@@ -24,10 +24,10 @@
 
 	if (self == Py_True)
 		s = true_str ? true_str :
-			(true_str = PyUnicode_FromString("True"));
+			(true_str = PyUnicode_InternFromString("True"));
 	else
 		s = false_str ? false_str :
-			(false_str = PyUnicode_FromString("False"));
+			(false_str = PyUnicode_InternFromString("False"));
 	Py_XINCREF(s);
 	return s;
 }