gh-141376: Fix exported symbols (GH-141377)

* gh-141376: Fix exported symbols

* _io module: add "_Py_" prefix to "spec" variables. For example,
  rename bufferedrandom_spec to _Py_bufferedrandom_spec.
* typevarobject.c: add "static" to "spec" and "slots" variables.
* import.c: add "static" to "pkgcontext" variable.

* No longer export textiowrapper_slots