htmlescape: inconsistency between _py_htmltext and _c_htmltext

There's an inconsistency between the definitions of htmlescape() in the 
Python and C versions. The Python version uses "isinstance(s, htmltext)" 
to test for htmltext instances. The C version uses "((v)->ob_type == 
&htmltext_Type)" which mirrors the old "classof(s) is htmltext" 
behaviour. Thus, the C implementation does not recognize subclasses of 
htmltext.

To allow for htmltext subclasses to work as expected, could 
_c_htmltext.c be modified so that:

#define htmltextObject_Check(v)    ((v)->ob_type == &htmltext_Type)

is replaced with

#define htmltextObject_Check(v)    (PyType_IsSubtype((v)->ob_type, 
&htmltext_Type))

Thanks,
Graham