Add const to several API functions that take char *.

In C++, it's an error to pass a string literal to a char* function
without a const_cast().  Rather than require every C++ extension
module to put a cast around string literals, fix the API to state the
const-ness.

I focused on parts of the API where people usually pass literals:
PyArg_ParseTuple() and friends, Py_BuildValue(), PyMethodDef, the type
slots, etc.  Predictably, there were a large set of functions that
needed to be fixed as a result of these changes.  The most pervasive
change was to make the keyword args list passed to
PyArg_ParseTupleAndKewords() to be a const char *kwlist[].

One cast was required as a result of the changes:  A type object
mallocs the memory for its tp_doc slot and later frees it.
PyTypeObject says that tp_doc is const char *; but if the type was
created by type_new(), we know it is safe to cast to char *.
diff --git a/Modules/_bisectmodule.c b/Modules/_bisectmodule.c
index 0d29462..e40297d 100644
--- a/Modules/_bisectmodule.c
+++ b/Modules/_bisectmodule.c
@@ -40,7 +40,7 @@
 	int lo = 0;
 	int hi = -1;
 	int index;
-	static char *keywords[] = {"a", "x", "lo", "hi", NULL};
+	static const char *keywords[] = {"a", "x", "lo", "hi", NULL};
 
 	if (!PyArg_ParseTupleAndKeywords(args, kw, "OO|ii:bisect_right",
 		keywords, &list, &item, &lo, &hi))
@@ -70,7 +70,7 @@
 	int lo = 0;
 	int hi = -1;
 	int index;
-	static char *keywords[] = {"a", "x", "lo", "hi", NULL};
+	static const char *keywords[] = {"a", "x", "lo", "hi", NULL};
 
 	if (!PyArg_ParseTupleAndKeywords(args, kw, "OO|ii:insort_right",
 		keywords, &list, &item, &lo, &hi))
@@ -137,7 +137,7 @@
 	int lo = 0;
 	int hi = -1;
 	int index;
-	static char *keywords[] = {"a", "x", "lo", "hi", NULL};
+	static const char *keywords[] = {"a", "x", "lo", "hi", NULL};
 
 	if (!PyArg_ParseTupleAndKeywords(args, kw, "OO|ii:bisect_left",
 		keywords, &list, &item, &lo, &hi))
@@ -167,7 +167,7 @@
 	int lo = 0;
 	int hi = -1;
 	int index;
-	static char *keywords[] = {"a", "x", "lo", "hi", NULL};
+	static const char *keywords[] = {"a", "x", "lo", "hi", NULL};
 
 	if (!PyArg_ParseTupleAndKeywords(args, kw, "OO|ii:insort_left",
 		keywords, &list, &item, &lo, &hi))
@@ -233,4 +233,3 @@
 
 	m = Py_InitModule3("_bisect", bisect_methods, module_doc);
 }
-