bpo-46417: Clear Unicode static types at exit (GH-30806)

Add _PyUnicode_FiniTypes() function, called by
finalize_interp_types(). It clears these static types:

* EncodingMapType
* PyFieldNameIter_Type
* PyFormatterIter_Type

_PyStaticType_Dealloc() now does nothing if tp_subclasses
is not NULL.