subtyping a builtin type in a C extension

[shawn]

I am trying to make a subtype of a string. Initially it will have no
new methods or attributes, the C equivalent of:

class myStr(str):
    pass

I have experimented a bit, but am currently making a mess of it. Does
anybody have an example they can point to of inheriting from a builtin,
or a suggestion to point me in the right direction?