Fixes bpo-29680: Older gdb does not have gdb.error. (#363)

This change is required to make python-dbg.py compatible with GDB versions before 7.3.

Tools/gdb/libpython.py

@@ -715,7 +715,7 @@ class PyDictObjectPtr(PyObjectPtr):
         try:
             # <= Python 3.5
             return keys['dk_entries'], dk_size
-        except gdb.error:
+        except RuntimeError:
             # >= Python 3.6
             pass