bpo-35132: Fixes missing target in gdb pep0393 check. (GH-11848)

Misc/NEWS.d/next/Tools-Demos/2019-03-04-02-09-09.bpo-35132.1R_pnL.rst

+Fix py-list and py-bt commands of python-gdb.py on gdb7.
\ No newline at end of file

Tools/gdb/libpython.py

@@ -1178,7 +1178,7 @@ class PyUnicodeObjectPtr(PyObjectPtr):
     def proxyval(self, visited):
         global _is_pep393
         if _is_pep393 is None:
-            fields = gdb.lookup_type('PyUnicodeObject').target().fields()
+            fields = gdb.lookup_type('PyUnicodeObject').fields()
             _is_pep393 = 'data' in [f.name for f in fields]
         if _is_pep393:
             # Python 3.3 and newer