Add __builtins__ to globals argument of py::exec and py::eval if not present (#2616)

* Add __builtins__ to globals argument of `py::exec` and `py::eval` if not present

* Refactor into inline ensure_builtins_in_globals function

tests/test_eval.py

@@ -25,3 +25,11 @@ def test_eval_file():
     assert m.test_eval_file(filename)
 
     assert m.test_eval_file_failure()
+
+
+def test_eval_empty_globals():
+    assert "__builtins__" in m.eval_empty_globals(None)
+
+    g = {}
+    assert "__builtins__" in m.eval_empty_globals(g)
+    assert "__builtins__" in g