This is the first of several precursors to storing tp_subclasses (and tp_weaklist) on the interpreter state for static builtin types.

We do the following:

* add `_PyStaticType_InitBuiltin()`
* add `_Py_TPFLAGS_STATIC_BUILTIN`
* set it on all static builtin types in `_PyStaticType_InitBuiltin()`
* shuffle some code around to be able to use _PyStaticType_InitBuiltin()
    * rename `_PyStructSequence_InitType()` to `_PyStructSequence_InitBuiltinWithFlags()`
    * add `_PyStructSequence_InitBuiltin()`.