bpo-23896: Add a grammar where exec isn't a stmt (#13272)

https://bugs.python.org/issue23896
diff --git a/Lib/lib2to3/pygram.py b/Lib/lib2to3/pygram.py
index 919624e..24d9db9 100644
--- a/Lib/lib2to3/pygram.py
+++ b/Lib/lib2to3/pygram.py
@@ -36,5 +36,8 @@
 python_grammar_no_print_statement = python_grammar.copy()
 del python_grammar_no_print_statement.keywords["print"]
 
+python_grammar_no_print_and_exec_statement = python_grammar_no_print_statement.copy()
+del python_grammar_no_print_and_exec_statement.keywords["exec"]
+
 pattern_grammar = driver.load_packaged_grammar("lib2to3", _PATTERN_GRAMMAR_FILE)
 pattern_symbols = Symbols(pattern_grammar)