Fix some stubtest complaints before they happen (#9585)

Add missing objects to various stubs

stdlib/lib2to3/pygram.pyi

@@ -1,3 +1,4 @@
+import sys
 from lib2to3.pgen2.grammar import Grammar
 
 class Symbols:
@@ -110,4 +111,6 @@ class pattern_symbols(Symbols):
 
 python_grammar: Grammar
 python_grammar_no_print_statement: Grammar
+if sys.version_info >= (3, 8):
+    python_grammar_no_print_and_exec_statement: Grammar
 pattern_grammar: Grammar