store_subscript precedence fix and...

Allow format specifier "%p" to indicate a nonterminal name,
like "%c" allows.

store_subscr -> store_subscript to match Python AST a little closer.

test/simple_source/expression/04_subscript.py

@@ -0,0 +1,11 @@
+# From 3.6.8 idlelib/query.py
+# Bug was handling parens around subscript
+
+# RUNNABLE!
+a = {'text': 1}
+b  = {'text': 3}
+for widget, entry, expect in (
+        (a, b, 1),
+        (None, b, 3)
+        ):
+    assert (widget or entry)['text'] == expect