Replace PyUnicodeObject* by PyObject* where it was irrevelant

A Unicode string can now be a PyASCIIObject, PyCompactUnicodeObject or
PyUnicodeObject. Aliasing a PyASCIIObject* or PyCompactUnicodeObject* to
PyUnicodeObject* is wrong
diff --git a/Include/unicodeobject.h b/Include/unicodeobject.h
index 4e492dd..592f4dd 100644
--- a/Include/unicodeobject.h
+++ b/Include/unicodeobject.h
@@ -1828,7 +1828,7 @@
 #ifndef Py_LIMITED_API
 /* Externally visible for str.strip(unicode) */
 PyAPI_FUNC(PyObject *) _PyUnicode_XStrip(
-    PyUnicodeObject *self,
+    PyObject *self,
     int striptype,
     PyObject *sepobj
     );