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 *.

attr, and copy_reg.safe_constructors.

Check return values after memory allocation.
Also use Py_True instead of PyInt_FromLong(1) for bool value.

Backport candidate.

Use integers in stat tuple, optionally floats in named fields.

[ 526072 ] pickling os.stat results round II

structseq's constructors can now take "invisible" fields in a dict.
Gave the constructors better error messages.
their __reduce__ method puts these fields in a dict.

(this is all in aid of getting os.stat_result's to pickle portably)

Also fixes

[ 526039 ] devious code can crash structseqs

Thought needed about how much of this counts as a bugfix.  Certainly
#526039 needs to be fixed.

[ 496873 ] structseqs unpicklable

by adding a __reduce__ method to structseqs.

Will also commit this to the 2.2.1 branch momentarily.

overrun avoidance.

Jack Jansen on python-dev.
Add simple test case.
Move vereq() from test_descr to test_support (it's handy!).