Revert "Changing lib.pyx to highlight an issue with astype_unicode (not a fix)"

This reverts commit fe8f645.

Author: ThibTrip

pandas/_libs/lib.pyx

@@ -577,10 +577,8 @@ def astype_unicode(arr: ndarray, skipna: bool=False) -> ndarray[object]:
     for i in range(n):
         arr_i = arr[i]
 
-        if checknull(arr_i):
-            arr_i = "is_null"
-        else:
-            arr_i = "not_null"
+        if not (skipna and checknull(arr_i)):
+            arr_i = unicode(arr_i)
 
         result[i] = arr_i