Fixes bpo-29680: Older gdb does not have gdb.error.

Author: abalkin

Tools/gdb/libpython.py

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